As to whether entering a course with lower than published grade requirements affect cohort standards: assuming we are talking about an established department that has a reputation and standards to maintain, it depends whether the published grade requirements accurately reflect base entry standards, rather than a crude way to thin out competition.
If the required grades are above actual base standards eg 3A*s asked but actually 3Bs (usually in specific subjects) will be adequate, then there is more scope for flexibility. With A levels in the recent past possibly lending themselves to being taught to the test, rather than being an indicator of understanding and application, the better places use entrance tests to flush out aptitude and base standards. If you got through these, and had a bad day, just missing a grade, it would be reasonable to be taken on.
So entry grades may not affect cohort standards, provided there is both learning potential and capacity to deliver the standards. If it seems 'unfair' that places may turn out graduates of different standards, that tends to be exposed by employers who also use cognitive tests. The 'bizarre' or 'weird' behaviour by universities and employers is simply down to their having 'seen it all before' and knowing how things work out. Bums of adequate calibre will always be welcome on empty seats.