What Cambridge don't tell you, is that their claim for number of candidates per place is actually an average of all colleges. It doesn't explicitly say this on the website, but the candidates per place is actually very different for different colleges. Yes, it's true that you won't get in with bad grades if you apply to the least popular college, but the number of people that you compete with at low-end colleges WILL affect the probability of that college accepting you.
If you've got good grades (not bad, not the best possible either), then you are better off applying to a place like Girton. If you meet the criteria, and if you they see that you have reasonably good/average grades for Cambridge offers, then you are much more likely to get an offer than if you applied to a college like Trinity. For example I heard someone (this year) applied to a very popular college (think it was Christ's college, can't remember), and another applied to Girton. The person applying to Girton was 1 of only 3 applicants for one place - and he got in. The other person applying to Christs didn't get in, because the competition was much more fierce.
People will disagree with me, and say that there is no difference, but this is the actual reality - if the college 'likes' you enough to give you an interview, then you are much more likely to get in if you apply to a lower-end college. Cambridge just doesn't tell anyone this because they want an even distribution between colleges.
And yes, the college can indeed see everyone's stats (from other colleges as well), but the college will still at the end of the day be reviewing all of the candidates applying there. Fewer direct applicants will equate to less of the 'best candidates'. When the human AT is looking at the list of candidates they HAVE to extensively consider, they will not see as many candidates (as say, Trinity), who have 98% UMS and 80%+ admission assessment scores. All candidates of a college are compared to each other. Fewer candidates means less competition.
This is why at Robinson college, it is actually 58% of applicants who get an offer (2017 data), where as for Christs it's more like 20-30%.