Ryanair and carry on baggage sizes.

Right so. Flew today out of Dublin. Bought a priority ticket to get the carry on bag.

Bought a new carry on suitcase last weekend specifically for the flight. Went to 2 shops searching for one that meets the dimensions, and picked up a hardcase. The label said the dimensions of the case are 55x35x20 cm, and measured it myself to confirm these were correct.

Ryanair's priority cabin bag dimensions are 55x40x20 cm. The bag is packed but not crammed with stuff so it's not bulging, but still it won't fit in the sizer at the airport gate and I have to fork out €70 to check it into the hold. The gate agent's opinion is that it's sticking out the front of the sizer. The large cabin bag sizes are essentially a box 55 cm high, 40cm across, and 20cm deep that is completely open at one side.

What do you think? Are the ryanair sizers deliberately smaller than their advertised size allowance? Has anyone ever brought a measuring tape with them to the airport?