In other words, if you are not getting fired and the company isn't folding, and that neither is happening anytime soon, then everything is just "business as usual" and missing a deadline here and there is just part of the natural statistical fluctuations that everyone have come to accept according to some unwritten rules.
I would go as far as drawing parallels between "being in the zone" for a software engineer and "meeting sales targets" for a salesperson. In that sense, not being in the zone every now and then is just as reasonable (or unreasonable, depending on your employer) as missing sales targets every now and then.