For context, this is how Flaming Durtles calculates the percentages at the end of each session:
First of all, only the items actually quizzed in the session you are finishing are counted. FD does not look at any past review results.
Then, FD counts the number of items in the session for which no mistakes were made. That is, both reading and meaning questions were answered correctly on the first try.
Then, that number is divided by the total number of items in the session, and shown as the “Overall percentage correct”.
And finally, the “Overall percentage incorrect” is simply 100 minus the “Overall percentage correct”.
So if you had a session with 10 items, and you made one meaning mistake, your overall percentage correct is 90%. If you had a session with 10 items, and you made three meaning mistakes and two reading mistakes on a single item (and the other 9 items were correctly answered every time), then the overall percentage correct is still 90%. If you make one meaning mistake and one reading mistake for two different items, your percentage correct will be 80%.
This is how it’s been calculated since the first release of FD, and I don’t think there are any bugs in that piece of code. I don’t remember why I’m calculating the percentage exactly like that, but it must have been to duplicate the calculation on the site itself. Or maybe a popular userscript.