Blue-collar and part-time workers tend to get paid hourly, daily or weekly; white-collar jobs are usually paid monthly. In a leap year, the income of a monthly-paid worker remains the same as in a non-leap year, meaning that the extra day is effectively unpaid labour; whereas someone paid by the hour, day or week will of course get extra money for the extra day.
I would like to say that the universe arranged this as a kind of redistributive fiscal policy, but sadly the extra money raised by the unpaid labour of the middle-classes doesn't go to the less well off - except, perhaps, to a small extent, in the case of public-sector workers. It's trickle-up economics, as usual. The solar system is a Tory - but I suppose we already knew that from Ulysses:
And therefore is the glorious planet Sol
In noble eminence enthroned and sphered
Amidst the other; whose medicinable eye
Corrects the ill aspects of planets evil,
And posts, like the commandment of a king,
Sans cheque to good and bad: but when the planets
In evil mixture to disorder wander,
What plagues and what portents! what mutiny!