There is a formula for calculating this in the notes; it shows that (skill bonuses + artifact bonuses) in percentage terms increase the total movement, which includes the additional movement from visiting different "troughs." So, you noticed everything correctly.
Справочник по НоММ-4 http://handbookhmm.ru/pamyatka-dlya-heroes-iv.html
Памятка по НоММ-3 http://handbookhmm.ru/pamyatka-dlya-homm-iii.html
и HotA http://handbookhmm.ru/vyishla-obnovlennaya-pamyatka-dlya-homm-iii-hota.html

