>>96528385
>those are modifiers for difficult terrain
No, modifiers for difficult terrain are an increase in cost of travel, either ×2 or ×3. You could see it as as a reduction of speed to 1/2 or 1/3, but it would make the calculations harder if the party is travelling though hexes with different terrain costs. (And 1/2 or 1/3 is not 2/3 or 1/3.)

>>96528388
>what i understood from the gazeteers
Indeed, the first Gazetteer was published in 1987. So if we're talking about the Known World in B/X, it's a different geological era. Here's how the Karameikos map still looked like in 1981 and 1983.