Not always! A few weeks ago enroute to LAX I got stopped at EDI in Gp1 when boarding as I was about 50cm over which I genuinely didn't realise.

I had to take an item from my small rucksack and repack it in my carry on which is OK in the sizer so it can happen. The GA was a rather surly person and I don't say that lightly and ignored the lower groups, some of whom had 3 BIG bags many of whom walked past her and when I politely pointed this out for consistency reasons she just ignored them and walked away from me!!

However, technically I was in the wrong so I accept that and I was 2 inches over to start with. I now have a smaller rucksack to carry so it won't happen again.