Bradford's 1-0 win at home to Burton leaves the eighth-placed Spireites four points adrift of the play-off party heading into the final weekend of the season - something Cook puts down to his side's inconsistency
"We're hugely disappointed," he said. "But when the dust settles and we have the season laid out before us it's plain to see. We've just been too inconsistent. That's the downfall.
"On any given day we've been a good side, we've looked a good team. But league tables don't lie, they never do.
"You might talk about a bit of luck, or one or two decisions. But that's just how it is."
Chesterfield grabbed the lead at Fleetwood against the run of play when substitute Armand Gnanduillet headed home Conor Townsend's corner in the 44th minute.
And from the restart the visitors doubled their tally, Marc Richards on the end of a long ball before firing a stunning 25-yard effort which was too good for Scott Davies - capping a stunning finish to the first half.
Richards then made it three-nil just after the hour mark, lashing home the rebound after Davies saved Sam Togwell's goalbound effort.
Andy Mangan's first goal since August gave the home side hope, a perfectly timed run to get on to David Ball's defence-splitting pass and a cool finish, but it was not to be.