Safe Date

Snow again this morning; Mother Nature's April Fools I suppose.
I'm puzzling over the whole "Average Last Frost Date" and "Frost Safe Date" thing. With many times more snow events this Winter than in any I can recall, I feel my weary body numbly slogging towards April 15 (the supposed Safe Date for Seattle) as though it might actually offer some physical sanctuary. I mean theoretically it's all over by then. No more scraping the windshield with a credit card while cursing my baffling inability to purchase a legitimate ice-scraper; no more storm systems piled up on the coast colliding with cold Canadian air masses like drunk party guests vying for the bathroom; no more staring blankly at the frozen tundra of a client's garden wondering if I should pursue my lifelong dream of doing anything but this. After the 15th, Winter just throws in the towel and starts getting fat for the off-season, because that's the Safe Date, right?
Well, however much faith you place in your given frost safe date, I'm willing to bet that someone somewhere cranked through some serious high-school level statistics in order to toss out that date with any measure of certainty. Accordingly then, someone somewhere must have applied similar statistical analyses to arrive at the average last frost date (unless these people, wherever they are, got drunk and are just throwing darts at a calendar even as we speak). Now, if I understand averages correctly (and thanks to 16 years in public education, I think I do) this average last frost date should fall somewhere in the middle of the range of possible last frost dates. So for every year like this year (and last year, come to think of it) where we have snow in April, there should be another year where Winter ducks out early to go fishing, say sometime in early March. For an impatient gardener or farmer this means that every year a wager must be placed, the stakes of which are whatever frost-tender plants or seedlings you throw into the fray of high-school statistics, weather patterns and drunken darts. Last year, I placed my bets on late March and consequently lost my whole first batch of vegetable starts. This year, so far, I've had more patience and kept my starts inside; as a result, I have pole beans growing up and through my window blinds. If this pays off because of an exceptionally late frost, then next year I'm doubling down and shoving everything outside in mid February. That's how the Average Last Frost Date works, right?