100x stronger is a lowball. Yamcha/Kuririn went from Large Building level or City Block level to City level, a difference of more or less a million times, with Tenshinhan going from either the same or maybe Multi-City Block level to at the very least the same tier. Ki-wise though, none of them probably even doubled their Ki when considering the battle powers at the start of Z.
As for the explanation why, the most likely one is due to the lead they already had over the Goku from 3 years ago before going on a similar training regime as well as greater experience in honing their Ki from both battles and observation, not to mention it being suggested they didn't split off on separate paths immediately. It's not really any different from them having gone from City or Island level at the start of Z to Moon/Small planet level after their training with Kami + an extra 6 months of solo training.
Hector said:
Old Piccolo tanking KHH with less than half his chi etc
He said he was using less than half his power, but didn't specify he meant half his Ki, something that definitely doesn't add up when taking into account the battle powers that would be revealed and the fact there's a huge discrepancy and lack of linearity between Ki/Battle Powers and destructive capability or physical strength.