It’s customary in many (most?) circles not to dissect the bill at the end of a restaurant meal to ensure everyone pays for exactly what they ordered. Certainly it is in my circles. We just don’t do that beyond ‘oh, so and so isn’t drinking so they can chuck in X amount less.’. I assume in the OPs circle it’s like that. Is far from unusual.
It's only customary in circles where people don't take advantage, most people I know do this, obviously some weeks you'll be up and some weeks you'll be down, it works out pretty even over time. It becomes pretty annoying though if people want to split the bill then they make sure they order more than everyone else, or they want to 'do rounds' but they magically skip their round etc etc, or they turn up to parties, don't bring anything with them, and drink and eat as much as they can.
I've got one in my family, and it is fucking tedious. I just can't do the above with them as they massively take the piss. For our wedding we went for a lovely pub meal in a village after, we were paying for everyone's food but we also decided to cover the bar bill to make it easier for the staff, when this relative found out, they told their family and they started ordering cocktails and downing them like they were going out of fashion at £10 each (or leaving them if they didn't like them), I knew they would do something like it and factored it in, but still.. there really are people out there who are like this, some are more subtle than others, but I think it is like a game to some people, to see what they can get for 'free' out of other people's generosity.. they never put their hand in their own pocket for anyone.
I wouldn't choose to socialise with people like that, but unfortunately mine are family, I suspect OPs friends may be a level of this, they are happy to accept generosity, and happy to bill split when they make sure they order more, but are not happy for it to go the other way, it can come as a shock to realise the friends that you like spending time with are only interested in a one way street that benefits them.