Double precision from ieee754 is 64bit, of which 53bits are the significand. X87 uses an 80bit format with functionally a 63 bit significand (they actually use

Double precision from ieee754 is 64bit, of which 53bits are the significand. X87 uses an 80bit format with functionally a 63 bit significand (they actually use a 64 mantissa, but for that actually doesn’t gain you anything, and adds many terrible edge cases). They use double precision values because that presumably provides more precision than they need, but 32 bit float would be woefully inadequate. They could use more (x87 or software float for instance) but there’s not really an any advantage to the increased precision you get, and there are many downsides.