It could be that your other gear isn't 100% accurate either, but you've compensated for the error (either knowingly or not) within your exposure/development regime. Say you were half a stop wrong with your other meters and half a stop wrong with your new meter... that adds up to a full stop very quickly.

People who calibrate their exposure/dev regimes often forget that they may only be internally cohesive, and comparing it to another photographer's regime may throw up quite large differences.