For example, by yourself, not on a track, no spikes, no coach, no one watching
vs
on race day, with competition, spikes, coach, etc.
More specific example:
I ran a 13:00 2 mile by myself today but I think I can do sub 12 on race day. (Haven't ran it yet but my mile is PR 5:31 on a track)
However, by myself, the first split of 6:15 felt hard like a race mile, but it wasn't anywhere close to my normal mile.