Why is tennis scoring so weird?

Dear Cecil,

I have a follow-up question to your answer regarding the scoring used in tennis. My question revovles around the use of 40 rather than 45. You state “the four points when 15, 30, 45 (which we abbreviate to 40) and 60, or game”, but do not go into more detail why 45 is abbreviated to 40. Any more insight on this? Mike G., Phoenix

Wikipedia’s tennis entry says that

Just so other folks know what column you’re referring too it’s

Why is tennis scoring so weird?