Long time NHRA fan here, I'll give this a stab.
For all of the 25 years I remember watching racing, there was always a 1% backup rule for a national record. That is for ET and speed both. It is to prove the run is not a fluke. Now, a lot of these really fast/quick runs ARE, because each race has one qualifying session held on Friday evening when the weather is cool and the track gets really good. This was the case with Capps. Only Friday night's awesome conditions could help create that run. Too warm and sunny the rest of the event. The 3.964 win be the QUICKEST run (until, well, it's not), but it's only a record if it's backed up. That also applies if run during qualifying or eliminations. Your 1% slower run has to come anytime during the event....ie, if they run late Sunday night and you lay down a monster pass. Capps could run *quicker* than the 3.964, and have it be backed up by the previously run 3.964.
Make any sense?
Just because I'm a family raising, meal cooking homemaking wonder, doesn't mean I'm a desperate housewife.