hmmm...... lets do the maths here. 55cmx40cmx20cm = 44000 cubic cm. Which, as I remember from school, is 44 litres.
Any bag with those measurements will be 44 litres. Except that's where it goes wrong, because the airline rule says the bag has to fit inside a box with those measurements so the bag itself will take up some of the 44 litres. And it's got rounded corners which reduce capacity. So either it's too big or it doesn't have that capacity.
Because it's mainly only thin polyester, if they are the external measurements you can probably get close to 44 litres inside but if it's stuffed full the dangly bits on the outside will push over the limits and stop the bag going into the frame at checkin in, even if the bag holds it's shape and doesn't go lumpy. The rule says that all the bag including external parts - including handle and buckles - has to go in to the frame. So try and fill this up to 44 litres and you could be in trouble - especially with Ryanair since those measurements are right on their limit, others have a long side length of 57cm which would at least leave space for the handle.
The price seems to be a genuine discount, other places are selling it for £25-27 but don't try stuffing it to the limit!