I can see how the low time guys could have problems after a year if they never had a chance to get the fundamentals down before they went to an airline. Maybe they squeaked through the initial but hand flying instrument skills and flying fundamentals were never that great - say they got by with help from captains and automation for the first year and it might bite them during the probationary PC. And as said above this is a good time for the company to weed out people they don't like.