A quick search of the Ross-tech wiki has confirmed my vague recollections of this issue:
http://wiki.ross-tech.com/wiki/index.php/00955
Basically in HP's case, if the key buttons have been pressed too many times whilst outside the receiving range of the car, then the rolling codes get out of sequence. Doing the manual sync by doing what you did basically resets the sync and explains why it's now working.
All it needs is, for example, small hands getting hold of the key and having fun pressing the buttons to lose the sync.
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