Unlike hard drives and SSDs which as you have observed are incessantly manufactured in powers-of-ten mega/giga/terabytes but marketed as if they were powers-of-two mebi/gibi/tebibytes, a RAM chip’s capacity is absolutely, definitely, 100% down to the individual bit precisely the capacity at which it is rated in powers-of-two megabytes. Due to the way that memory is accessed there is no other way and it cannot be fudged. (The exception is ECC RAM which typically has an extra bit per byte to hold the parity data, but this is not accessible to the user so that’s moot.)

There is a small bite of your memory space taken out for Memtestx86 to reside in, which is necessary in order for it to run. Your BIOS probably has some portion of memory reserved as well, either for peripheral memory mapping or for use as video memory, or similar.

There is probably also some rounding going on in the total capacity that Memtestx86 reports.