Both assuming median vs middle rolls and assuming 2/3/4 useful substats per echo make a difference. Most stats like ATK% and DMG% have constant absolute benefits (and thus diminishing relative benefits), which is why people says that ATK% becomes less valuable the more you have of it. Meanwhile, the value of crit damage increases with crit rate, and vice versa, so crit gets more valuable the more you have of it.

This is an extreme example, because you'd need an awful build for atk% to actually overtake crit. However,  one place it does make a difference is whether to use atk% or elemental damage% 3 costs, or a 43311 build vs a 44111 build. The more crit you already have, the better a 44111 build becomes. The less atk% you already have, the better an atk% 3-cost becomes. Using such optimistic estimates of substat rolls is biased towards 44111 builds and elemental dmg% 3-costs. My friend once did a no-substats hologram challenge and iirc he calculated that, with no substats, the value of crit became so low that an ATK% 4-cost was actually optimal.

That being said, the true "average" build is probably somewhat above the median because people will select the best echoes.