Because universities are hilariously bad at deciding levels of entry for Access to HE Diploma applicants.
Example 1: I am currently studying an access course in electronic engineering, I am looking at going into computer science at uni. I have an offer from KCL for 33 distinction, 12 merit (I think it's a fair enough offer). I also have an offer from bath (a university of about equal reputation) for 36 distinction, 3 merit, the remaining 6 at pass. Why does Bath want 3 more distinctions but is willing to let me have 6 credits at a pass grade (a grade which I personally think is so easy to achieve my 14 year old brother could do it)? Who the hell knows.
Example 2: A classmate of mine applied to several universities for civil engineering (he's doing the civil engineering/built environment access course), all with varying offers. Brighton gave him an offer of something like 18 units at merit (just about the easiest offer to a year 1 entry in the country if you ask me), Southbank (definitely in the "close to but not quite at the bottom of the barrel" class) university gave him an offer of something like 15-18 distinction, 15-18 merit, the rest at pass. Kingston (not a great uni, let's be honest, about the same level as Brighton I think) won't even let him into year 1 entry with an access course, and will only let him onto a degree with a foundation year. Why? Like I said before, not a bleeding clue.
Example 3: Durham run a foundation programme, where students of varying abilities and experiences, mostly mature and international students, can get onto do degrees at Durham without A*s at A level. A man on the course I did last year got on it. His offer was to simply pass the course. I would say this makes sense, as most people who go on the course have been out of education for some time and often receive unconditional offers.
Southampton (a university with a similar reputation to Durham in its engineering department) also runs a foundation year programme for students who don't meet the specified requirements of A*AA at A-level. The A-level requirement is ABB, pretty fair I would say. However, if doing an Access to HE diploma, the requirement is ALL credits at distinction, no questions asked (this is the same requirement as OXBRIDGE to get on their normal degree). A girl on my course last year was so dedicated to getting on this course she actually did it! If it were me I would've told southampton to do one and go somewhere else.
As one can see, there doesn't seem to be any rhyme or reason to universities' Access to HE requirements (other than *most* top 30 unis having an entry requirement somewhere between 30 and 39 distinctions). The only conclusion that can be drawn in my eyes is that an embarrassingly high proportion of universities have no idea what the appropriate entry requirements for access course students should be.
You're more than welcome to draw your own conclusions.
May I ask how is UCL's offer 28 credits at distinction? Each unit at access is 3 credits, 3 doesn't go into 28, you can't get half a merit and half a distinction in an individual unit.