When you convert back from Fortnite to Apex, everything (ADS, scopes, Apex hipfire) is matched to the Fortnite hipfire sensitivity. And since Fortnite has a locked FOV of 80, everything is matched to this FOV. So you get a big difference compared to Apex, where your FOV is 110.
If you set everything in the conversion setup to MDV 0% you will get the same result both ways (except a small difference due to lack of decimals in Fortnite).
Apex to Fortnite
Fortnite to Apex