They didn’t mimic existing units, an imperial ton is close to a metric ton, and the spelling tonne is just an alternative spelling of ton. In some parts ton means imperial ton, and tonne means metric ton, but it’s not standardized. In German, where the word originally comes from, it’s Tonne (btw the e is not silent, it’s [ɛ] as in let. Or in Porsche (no, it’s not pronounced porsh…).)