Copied from the Honda Car Owners Club...

I think to put it more clearly, the original Honda program for implementing the VTEC engagement is that it is actually variable depending on a few parameters. Most people assume the VTEC to kick in on lets say XXXX rpm, but the ECU (lets reference with a stock ECU that is) doesn't ALWAYS kick in at that XXXX rpm. The ECU can decide NOT to activate VTEC if it senses that there is not enough engine load to justify the high-cam usage. For example, a ITR P73 ECU engages VTEC at 5700RPM (never sooner) under heavy load conditions (ECU already running in open-loop aka power mode, ignores O2 sensor.) BUT, if the ECU doesn't sense a heavy load (ECU running in closed-loop mode aka "14.7 A/F" mode, ECU reads O2 sensor to determine 14.7), the VTEC will only kick in no later than 6200RPM. So depending on how you drive, the VTEC can kick in anywhere from 5700 to 6200rpm.

That seems to be the best explanation I've found.

Edit... Here's the link: http://hcoc.2u.com.my/vtecpoint.htm