Well you might assume similar intake, but you may be wrong.
Dd1's at a comprehensive. They split the year into half for setting in year 8, 6 forms in each and set the maths in each half. It's random forms so no setting over the split.
As it happens this year the same maths teacher is taking bth top sets, I don't know if that's usual, or has just happened that way.
Dd1's top set has about 1/3-1/2 level 8 and beyond pupils (some of them did last year's GCSE papers and got high Bs), the other top set had one level 8. She'd have been half way down in the other top set.
In maths I doubt it's to do with marking, as it's much more absolute. In English and such like it's much more subjective.
What has happened in dd1's set is that there is a group of 4 of them that really get into the maths and are asking not just for more to do, but harder things to do. It's not just moved them on, but also the rest of the set, who see this as something they want to try too. So they have done harder work because they have finished what has been set and asked for harder stuff, the other class takes much longer to get a concept and then generally take longer to do the work too.
In the same way, I remember the year below me at secondary was considered to be realtively poor at maths compared to us. One of my friend's parents was really upset that she didn't get into the top set, and when asked why they said she'd have been in the top set usually but our year was generally mathematical and had a lot of very good mathematicians. This was clear when GCSE/A-level results came out.