According to John Hines :

Tongue & groove means the edge of the plywood, at the mill, has a 1/4"
cut in the top and bottom sides, leaving the middle 1/4" to stick into
the slot cut in the other edge of the next sheet of plywood.

[Or the reverse ;-)]

So if you have a slight gap between the sheet, but not enough to expose
the whole edge, you could measure just the top cut on the T&G.

It makes for a stronger joint to have the two pieces interlock.

Indeed.

It's also possible he's seeing a ripped off edge that doesn't quite meet,
and has been undercut by a chunk split out of the underside of the ply.

In any event, 1/4" floor sheathing would be more than flimsy. If you
stomped on it, there's a good chance of going through. You'd get a very
queasy feeling walking on it.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.