plotting – Cannot understand meaning of “identical”

This came up in the context of plotting solutions to NDSolve, but I’ve reduced it to the following issue. This really has me scratching my head.

points = {{0, 0}, {1, 1}, {2, 3}, {3, 4}, {4, 3}, {5, 0}};
ifun = Interpolation[points]

ifun is now an InterpolatingFunction. I cannot plot ifun, but I can plot ifun

Plot[ifun

If I make the following assignment, I cannot plot ifun2

ifun2

In other words, the following statement will not plot anything:

Plot[ifun2

However, if I do this:

ifun3[t_] = ifun

I can plot ifun3

ifun2

The answer is “True”. Similarly for ifun3