"They're not together, but there is a flaming reanimate and a ghoul in the next part." Eldrian wasn't sure if they were indeed there, outside of the undead they had not run into any other monsters. Thus it was relatively safe to assume only undead were in the mines as expected.
The rest were likely in underground tunnels near but not connected to the mines. Eldrian was not sure if this was the so-called subterrain, he had almost no knowledge of it. Regardless, it made things more confusing as some undead had ended up not being in their path.
Thus they could never be certain of the precise situation, always needing to prepare for the worst possible case. After all, it was not safe to assume that there would be fewer enemies than there could technically be- but likely won't be. This slowed their advance as the group prepared well before every fight. Also waiting to fully recover after every fight.
