Accumulated altitude Suunto 9 baro

Good afternoon all, trust you are doing good.

Does someone know, why my Suunto 9 baro shows incredibly big altitude numbers? Measured height (at starting point) as of 74 m is correct, but the accumulated altitude metres during the ride are too much (1316 metres of accumulated altitude for a 80 km ride, which is actually quite flat....).

Any ideas, pls?

Thanks, cheers,

Christoph