There are rounded N95 masks, too. The important part is probably the behind-the-head straps.
The greater force means a better seal. But that seal comes from your face deforming to fit a rigid mask, and your face would prefer not to be deformed.
It's better to use soft polyurethane or foam to seal around the edges, and that's what some P100 masks do. But because they have higher upfront costs, they have cultural associations with industrial work and gas masks, while people use disposable N95s for things like woodworking. Also, the typical design with 2 flat filters to the sides makes it harder to talk to people while wearing them than disposable N95s do.
Here is a typical disposable P100 mask. It adds a foam seal around the rim, and it's much more expensive than a N95 mask. I doubt the production cost is that much higher, and in theory, the foam seal could be separate from a replaceable filter.
The important part is probably the behind-the-head straps.
That's my guess too.
that seal comes from your face deforming to fit a rigid mask
Are you sure? It seems to me that even the most "rigid" masks I've tried are still not very hard, and with sufficiently tight straps while my skin deforms slightly the masks deform much more?
Here is a typical disposable P100 mask.
Note that this is a valved mask, so it probably wouldn't have done well in a source control comparison.
I wore duck bull masks the whole time, by halyard, it's what hospitals use. So comfortable, I can wear them all day pretty easily, I don't like force on my ears so having the elastic around my head is like it's not even there.
And very easy to breathe in, very little static pressure difference
Same for long flights, not bad at all
I've been thinking about trying to get a valve on it. I haven't done valve masks because I think it's more polite to help filter myself, but now that most people aren't wearing a mask (on the plane, etc, not in a hospital I mean), it'd be nice to have a valve so it's not so humid
I also had to buy the duck bills in bulk from a medically supplier and it's annoying to have to get another huge pack
Lai et al. just published a paper looking at how well various masks worked for keeping others from getting sick ("source control"). Their university press office summarized this as:
Study finds all masks effective, but 'duckbill' N95 masks far outperform others, suggests they should be the standard in high-risk settings
Now, I personally think duckbill masks are the best disposable masks: they're cheap, comfortable, fit me well, and are more breathable due to their larger area. Plus, as masks that manage to be unfashionable even by the standards of N95 masks, if they weren't better in non-fashion ways they wouldn't be on the market anymore. But the study didn't show that they're the best kind of N95, only that they do better than the other masks they tested, none of which were N95s.
The study compared five conditions: no mask, cloth mask, surgical mask, KN95, and (duckbill) N95. This isn't a study that can tell you anything about the differences between N95s!
To me the most interesting parts of the study were (a) they captured the viral RNA and measured viral load with qPCR instead of using bad proxies like particle count and (b) they found KN95s did way worse than you'd expect from their filtration efficiency:
The majority of the KN95 respirators used in our study (reported by an N95docon.org to have consistently high filtration efficiency but variable and high flow resistance) did not outperform loose-fitting masks and when including other brands, KN95s met inferiority criteria compared to cloth masks for total viral aerosol. One possible explanation is that we noted that the KN95 respirators we provided were relatively stiff and did not seal consistently along the entire perimeter of the mask. By contrast, the cloth masks brought by our volunteers tended to wrap farther around the face possibly providing better fit and lower flow resistance. We used one surgical mask brand for these tests so that result may not be representative of all masks; the same brand used in prior CDC-funded studies of masks for influenza source control. The relatively high flow resistance of KN95 filters, compared with surgical and cloth masks, combined with poor fit tended to promote leaks around the face seal.
I recall a lot of people (including us) using clips to convert the KN95's elastic ear loops into a behind-the-head attachment, for a much tighter-fitting seal. It would be interesting to see if that's enough to make up the difference!
(I wonder if this means that events that require "high-filtration" masks should switch to explicitly requiring N95s or better, now that those are widely available?)