I've never really looked at it systematically but I wonder whether
some olives are better than others, where 'better' might be 'softer',
so more likely to seat in the fitting. I just did a complicated
little run that I built up on the bench (fresh pipe, no alignment
issues, plenty of access to tighten everything) then installed and it
leaked in about half the joints. Took it off and recycled most of the
pipe and all of the fittings but, obviously, had to use fresh olives -
every joint was a good 'un. If I knew where I got that pack of olives
(probably Screwfix), I'd get some more while they're still on the same
batch!

On the other hand, it might have been a 'good day/bad day' thing and
nothing to do with the olives......