The 275-gallon IBC is one of those standards that everyone respects and nobody really set. There is no ISO number that says "an IBC shall be 275 US gallons." The number emerged, as so many do, from a pallet.

48" × 40" is the standard North American "GMA" pallet, which is itself a leftover from the Grocery Manufacturers of America specifying a wooden pallet size for efficient truck loading in the 1960s. It so happens that a roughly-rectangular HDPE bottle that fits snugly on a GMA pallet and stands about 46" tall holds around 1,000 liters — which, converted to US gallons, is 264.17. The industry rounded up to 275 because that's the convenient round number that still fits in the same bottle.

Europe landed on 1,000 liters — the honest number — and calls them "1000L IBC totes." Americans took the same physical container and gave it a different nominal volume, because we still use gallons. Both numbers describe the same container.

The 330-gallon cousin came along later as a taller version for industries that wanted more volume in the same floor footprint. The "330" is again rounded — actual capacity is closer to 330 liters × 3.78 = roughly 1,250 liters or 330.22 gal.

None of this is on any datasheet, but it explains why if you ask a tote manufacturer what 275 means, the answer is, "That's what we always called it."