That small difference

The other day, my older one brought home a picture he had coloured at his Daycare Centre. He was extremely proud of it, and justly so- every bit of area had been coloured, with even strokes, and the crayon had strayed beyond the outline only a couple of times. He had obviously taken a lot of care over the painting, and it showed.
Normally, good worksheets go on the doors of the cupboards in his room, but since there is no space left on the doors, and this particular worksheet was more than just good, I slipped it under the glass top of the dining table.
The first day, he returned to admire the picture every fifteen minutes, and it gradually tapered down to two or three times a day.

The next Friday, he handed me another picture he had coloured. It was neatly done, but nothing spectacular. Though most of the area had been covered, the strokes were uneven, almost bordering on careless. The crayon had gone beyond the outline a couple of times, and he hadn’t reigned it in fast enough. It would have been good two months back, but was just a shade over acceptable after the effort of the previous week.
“How is it, Mamma?”
The look in his eyes was enough to melt the heart of even the most demanding mother, “It is nice. But……(last week’s was much better)…”
He didn’t wait for me to finish. “Then put it here, Mamma”, he said, pointing to the space next to the picture of the previous week.
“No darling. This is good, but it is not that good”, I tried to tell him, but he would not listen.
“This is good. Teacher said so. Put it here, please.” The please was a slight afterthought, but he had used the word without prompting. And he was clearly devastated that I did not on my own display the picture.
“Okay, but only this time. From now on, only something that is really good will go here”, I told him as I slipped the picture under the glass.
“But this is good”, he insisted, even though I knew that he had not done it with as much care as he had the previous one.

How do you explain to a not yet five year old that even though the result may look nearly the same, there is a huge difference between giving something your best, and giving something just enough. And how do you deal with him till he learns?