It becomes a scam if reasonable leeway is not allowed. For example 10 kg means < 10.5 kg. If the airline means to cut it off at exactly 10 kg they should use the proper number of significant figures in the terms: 10.0000 kg or whatever. Stating the limit as 10 kg implies rounding to the nearest kg.
MAXIMUM generally means the uppermost limit. So if Ryanair have a 10kg MAXIMUM, then thats what it is.
Oh, they do have plenty leeway. You can carry any weight you like BELOW their Maximum 10kg.
Nearly everyone, bar 3 or4 on each flight manage this. Thats about 98%. I certainly wouldn't want to show myself up as one of the 2% dafties that cant count or follow simple instructions