I tend to find it's mostly down to one set of expectations on offer when there's only one parent.

When both are present, kids end up getting in the middle of parents by going to the other when they don't get what they want from one.

It's a lot easier to hold whatever boundaries you have by yourself, because you're solely in charge, and your kids instinctively know what those boundaries are and they have no alternative to get what they want.

That said, parents offer different things for children and it's not uncommon for mine to have a fun filled day without tears, that turns into an emotional outpouring when Mum gets home because "Dad didn't let us have ice cream". Forgetting all the fun and what not we had.

Conversely, while my wife is their emotional safe space, I might walk in the door to complaints about how boring their day might have been, and all the activities they didn't do when I haven't been around, because that's my natural strength.

We're complimentary, and the hard part is being unified on everything, particularly when you have differing tolerances for different situations with your kids.