If you're talking about the "Most Popular Boards By Activity" thing, then it displays the ratio of posts from this forum that you wrote

Right, but presumably it was 21% in meta, not 0.21%. Usually you calculate a precent by first calculating the ratio and then multiplying x100 and adding the % sign. It looks to me like they forgot the multiply by 100 step. Or maybe I'm wrong and it's only giving me info about 0.78% of my posts (that would be weird, though, more likely that this info covers 78% of my posts, right?)

You didn't get what I said

It's not

(number of posts you posted in this forum) / (number of total posts you posted everywhere)

It's

(number of posts you posted in this forum) / (number of total posts posted in this forum, from everybody)

Development & Technical Discussion has ( 108348 - 27558(subforums) ) = 80790 posts.

According to my stats, my ratio is 0.7711% and I posted 623 posts there.

And 623 / 80790 = 0.7711%