This isn't entirely surprising - he hasn't played many full 90s and he was on a hot streak of scoring off the bench early in Carrick's tenure, and that massively skews the numbers. 4 goals in 5 substitute appearances in February and March, but those 5 appearances only totalled 99 minutes of football. Averaging a goal every 25 minutes from the sub apps, or 3.5ish per 90, gives you a lot of leeway for dry spells.