Mercury's action and conflicting accounts of the opposing ships

Why is it uncertain whether both larger ships were together when they engaged the Russian brig Mercury?

The sources disagree, and ships described as operating together could still have been separated by miles rather than hundreds of yards.

Conflicting surviving accounts leave the ships' positions uncertain. Describing two ships as operating together does not establish their precise distance apart. Several miles of separation would allow an engagement to develop before the other ship joined it. A separation of only a few hundred yards would imply a different tactical situation.