I’m not sure which form is actually more common, but if you know that the source kanji for そ is 曾, it’s not all that surprising. The horizontal stroke you see is really just the connected form of the two little strokes at the top of 曾, so if you choose to write them one at a time instead… you get the kana you’re asking about.
If you want, you can take a look at this video I shared a while back about how the hiragana evolved from their source kanji to get an idea: