They will put your bag in their sizer
They MAY put your bag in their sizer. Depends on random factors (how full the flight is, how many other bags people are trying to squeeze on, how you and your bag appears to them, their mood, etc. - all things beyond your control). Some people take chances and skate through. Some people get nailed and have to deal with it one way or another.
Personally, I prefer to always be within the most strict limit I will encounter (OTOH my spouse typically exceeds the most strict limits, figures she can either sneak by or charm her way out of a jam, and usually does).
If your bag is not packed full, and really can be squeezed down to fit the most restrictive limit (when challenged on the size, it almost always goes into a sizer box gizmo so there's no argument), you should be OK - there's nothing that says you can't squeeze your bag a bit to fit it in. If this is true (and you can resist the temptation to fill it), you should have no troubles:
If it’s not packed fully, I’m confident it will fit easily
Weight limits are another story, though.