Open Water Anomalies

Does anyone else swim open water times that seem to be ... a bit faster than physically possible? Here's my dilemma: I swam the Big Shoulders 5K this past weekend at Chicago. My time was fast, REAL FAST, 1:10.04. Or to put it another way, if I swam a pool 1500m (an event less than one third the distance) at that pace, I'd shatter my PR by almost a minute, or if I swam a pool 800m at that pace, I would beat my most recent swim at LC Nationals by 8 seconds. Now, I believe in the power of positive thinking, and Total Immersion, etc., etc... But, this is so far beyond the realm of the plausible, I am not certain whether I should be crowing or questioning. It would be easy to assume that the course was simply measured a tad short, but this is not the first time I have had an open water swim bordering on fantatsy. Three years ago I swam a salt-water 3K in 40:20'ish, which again is a pace that would have shattered my 1500 PR, and is substantially faster than the pool 800 I swam 7 days before. I asked the meet organizers about this, and they assured me that this is the same, closely measured course they use for several open water swims each year, and that the added buoyancy from salt-water could account for the difference. I accepted that answer at the time, but now that I appear to have duplicated this feat in fresh water, I am wondering again. To get to the point: does anyone else find that they are prone to swimming in open water at a pace they cannot duplicate in shorter pool events? Does anyone have any ideas about what might cause this? Matt