it is logical that as a barrel wears out a looser fit between bullet and barrel will result in less speed.
While it may seem illogical, a "worn" barrel can also be "faster", though generally less accurate.
Logic says, "tighter bullet fit, better gas seal, higher velocity..."
But what this is leaving out is FRICTION. Tighter bullet fit means more friction, which can mean LOWER velocity.
It all depends on at what point in the barrel life/bullet fit you are at, when measuring.
When it comes to the gas seal, you have complete sealing, until you don't. When you get gas blow by, then you don't. Until then, you do, and when you do, a tighter fit doesn't get you anything but more resistance to bullet passage (friction) which can mean Lower speeds.
Its not any kind of a hard and fast rule, but it CAN happen, when conditions are just right.