You are right that per FTE is probably a better measure technically, but this just leaves the unions a loophole: they can just classify workers as something other than FTE. They are very clever at finding these sorts of loopholes and exceptions.
What they can't do though is lie to the tax office. That crosses a threshold from being misleading to being illegal.
There is a big incentive for unions to be misleading about what their members are paid. It's hard to garner a lot of sympathy from the public when the average member of the public is paid a lot less than the average union member. That's not to say that this is always the case - the wages, agreements and marketing to the public by the unions varies significantly between unionised industries.
What they can't do though is lie to the tax office. That crosses a threshold from being misleading to being illegal.
There is a big incentive for unions to be misleading about what their members are paid. It's hard to garner a lot of sympathy from the public when the average member of the public is paid a lot less than the average union member. That's not to say that this is always the case - the wages, agreements and marketing to the public by the unions varies significantly between unionised industries.