The Glovers were 1-0 up thanks to Miller's header when the former Man City forward was handed the chance to double their lead from the spot just before the break but his penalty clipped the crossbar on its way over the away end.
And Webster says it happens to the world's finest footballers.
"The best miss them," he said. "Ronaldo has missed a couple as well as Messi.
"It's one of those issues where you have to be brave enough to stand up and put your name forward to take it.
"He's practising them in training every day, and nine times out of 10 they are flying into the back of the net, but this time it didn't."