Everyone claims to be of nobility in Poland - but really, the vast majority were just peasants.
if nobility were 10 per cent of the population in the mid of 18th century then as people started to mix more (with a kickstart in the middle of the 19th century and hardly any notion of nobility as a separate caste after the WW2) there is a great probability you have some ancestors from former nobility - simply remember that before 19th century the nobility mostly kept to themselves (if one was of nobility they had both parents of nobility 95 plus per cent of cases) - then mix the population thoroughly and in a couple of generations you can have 100 per cent of people who can claim their ancestors where of nobility - capisci??? - this is actually very similar to what actually happened
another issue is how come 10 per cent of the population were nobility - well nobility was not a case of owning a land only of owning a title - there were multitude of 'farmer-nobles' who worked their own land in certain areas of Poland or the Commonwealth (Podlasie, big swaths in Lithuania proper and Lithuania sensu lato (modern day Belarus) - there were also multitudes of non-haves szlachta who owned no land and lived off the service to the magnates ('golcy') - this was part of the reason why magnates were so influential - they had plenty of voting supporters and in need armed force in their noble clients)
in Podlasie many villages were in time ennobled en masse for their valour in service to the king - this is one of the reasons why there were so many 'farmer-nobles'