I would imagine it's because selecting the "heated seat" option deselects the "sport seat" (which are also heated) option and the M-Sport package along with it.
Honestly though, the configurator is an annoying setup at best and I'd rather trust a proper build sheet with all the included option codes listed. Not to say that BMW package/line/option setups are logical, far from it sometimes, but the configurator isn't the best for definitive information that's for sure.
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