Total of request bytesIn is not equal to page bytesIn

[mems]

I get inconsistent values between page bytesIn and the total of requests bytesIn with Firefox and Chrome, but not with Edge or Safari mobile. On realistic websites (~2MB) the difference is around 15-25%. I don't see where the difference come from.

For example, if I read test data for https://example.com across different browsers, from the first run's firstView, I get:

Chrome (JSON):

Code:

view.requests.*.bytesIn = 769 + 672 = 1441
view.bytesIn = 9367// KO +7926

Firefox (JSON):

Code:

view.requests.*.bytesIn = 769 + 672 = 1441
view.bytesIn = 9715// KO +8274

Edge (JSON):

Code:

view.requests.*.bytesIn = 899 + 883 + 997 = 2779
view.bytesIn = 2779// OK no diff

iOS (JSON):

Code:

view.requests.*.bytesIn = 893
view.bytesIn = 893// OK no diff

Chrome Android (JSON):

Code:

view.requests.*.bytesIn = 1408 + 1300 = 2708
view.bytesIn = 2708// OK no diff

Firefox Android (JSON):

Code:

view.requests.*.bytesIn = not available
view.bytesIn = 10647// Can't check the accuracy. I suspect Firefox load something in addition to the page, see https://github.com/WPO-Foundation/webpagetest/issues/535

Does I miss something or it's a bug ?