The rest of the party could shift the treasure distribution towards the lower-leveled character to help them even it out, but yeah that's probably true in most cases.Also, math. Once you're past 8th level, you'll never catch up.
A 1st level fighter who joins a party with a 7th level fighter will only be 1 level behind, by the time the other fighter reaches 8th level.
But a 10th level fighter who joins a party with a 20th level fighter will always be 10 levels behind. When the new fighter reaches 20th level, the other fighter will be 30th level.