I don't know how true this is but I have heard Ryanair will use the absolute legal minimum amount of fuel whenever possible whereas other airlines might fly with a bit more.

In theory though that shouldn't matter because as you say, the legal minimum should really be enough.

That seems like a cost/convenience tradeoff: The implication of only carrying minimum fuel is that the pilots can't hold for long to see if conditions improve and instead have to immediately go for the alternate destination airport.

The consequence of that is everybody ending up in the wrong place, but not in an unsafe way.