There is always a little more toothpaste in the tube.

I have a question about the following underlined phrase. I know it is originally from Bill Bryson.

Could someone give me some clues to translate it?

Eventually, with reluctance, the supervisor nodded at the clerk and told him to check me in, but he warned me not to try anything as slippery as this again and then departed with his colleagues.

The check-in clerk issued me a boarding pass and I started toward the gate, then turned back, and in a low, confidential tone shared with him a helpful afterthought.

“There is always a little more toothpaste in the tube,” I said. “Think about it.”

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