The perceived rhythmic grouping of any keyboard figure need not coincide with either fingering group or pattern group. In fact, it seems to me that the musical effect is usually much more interesting if it does not. There is also the intriguing but common case of hearing something in a different rhythm to that in which it was conceived; rather like one of those isometric pictures which can be seen in two ways. A frequent instance is mentally mixing 3/4 with 6/8.
Perceived and intended rhythm have fascinated me since I was a kid, when I invariably felt all my time signatures "wrongly". I do not think that things "happen" in any particular formation, Tash; there are no rules about it. It is we who CHOOSE how we shall perceive anything we hear. The greater the variety of our rhythmic perception, the more meaning our listening and playing will contain, or so it seems to me. Even a simple arpeggio has an infinite continuity of perceived rhythm; if we dare to choose freedom over restriction.