Because he was an Earthling, his potential was limited by his innate talent. In the original work, Krillin had a Battle Power of 1083 when fighting Naba, with his maximum being 1700 at full strength. Therefore, setting Krillin's Battle Power limit at 1500 was not contradictory. At this time, Yueping's Battle Power was around 1400, so Earthlings had limited potential.
Namek Saga: His strength improved somewhat after fighting Vegeta, and he also trained mentally with Gohan aboard the spaceship for a while. Around 1500 (as detected by Sabot's scouter).
After the Great Elder unlocked his potential: 13,000 (official). His potential limit increased to roughly 100,000.
When fighting Frieza: 75,000.
Tenshinhan's Battle Power, meanwhile, was 1823. After all, he possessed Three-Eyed Human blood and was not a pure-blooded Earthling.
But with this aid, Krillin could continuously break through his own limits and eventually surpass Goku.
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