I just noticed that Jottacloud seems to be reporting its quotas on its website in Terabytes as a power of 10, so its 10TB is actually 10,000,000,000,000 bytes, as opposed to Tebibytes. Rclone about gives me 9.095 TiB for my account.
OneDrive use Tebibytes throughout so when you buy 1TB you get 1.005TiB as reported by rclone.
PCloud does the same as OneDrive. Not sure about other cloud providers, but this leaves Jottacloud as a bit of an outlier in using slightly misleading figures.