Why is military spending always defined as relative to GDP and not to total tax income, where the actual money is coming from? Because that’s how NATO defines

Why is military spending always defined as relative to GDP and not to total tax income, where the actual money is coming from? Because that’s how NATO defines it, such that each member makes a proportional contribution. It would be silly to expect say Estonia to match Germany in real terms, but it’s reasonable to expect Germany to match say Poland in relative terms. (Yes I know Sweden is not a NATO member but most European countries are so it makes sense to compare like with like)