kB, MB, GB, etc. were genuinely ill-defined when the drive manufacturers started using them to their advantage. Those units were not standardized by SI or ISO, there was precedent for both the binary and decimal interpretations, the unambiguously binary KiB, MiB, etc. had not yet been introduced, and industry standards organizations like JEDEC and IEC were not particularly helpful in resolving the situation.

None of those mitigating/confounding factors apply to RPM, so the current lies are more blatant.

(Though it does seem somewhat relevant to point out here that a "500GB" drive usually has an accessible capacity of 500,107,862,016 bytes, implying an effective definition of 1GB=1,000,215,724 bytes, which is neither 1024^3 bytes nor 10^9, nor 10^9 rounded up to the nearest whole (binary-sized) sector. And that effective GB definition varies with drive capacity.)