Agggh! I'm glad you pointed out that incongruency. When I was reformatting the graph, I only copied in half of the US Mean Contribuions so the first 4 rows of that were incorrect. The graph has been updated. The numbers are in fact a ratio of averages. LW mean contribution is calculated as follows: (Total Contributions of Subgroup)/(Total Income of Subgroup).

However, I am not seeing where you're getting that US-LW income is about the same as US-General; for 0-25k the USA-LW mean income is $11k, the USA-General is $15k.