I'm looking at some device data from https://skywater-pdk.readthedocs.io/en/main/rules/device-details.html#v-low-vt-nmos-fet say specifically VTXNN42L vs VTXNSN15L which shows that (given constant W), increase of channel length results in a lower Vt ... which I don't undertand. I thought increased channel length increased Vt but obviously I'm missing something ...

Reverse short channel effect (RSCE). Can be googled.

Thank you !