A single "u" does the work of "oo" or "ou". It is quicker to type, and it is still the same app.
Formal transliteration marks a long vowel with a line above the letter — ū — instead of doubling it. Drop that mark, which most keyboards cannot produce easily, and a single "u" is what remains.
The same shortening happens when people type quickly on a phone. The doubled vowel is the first thing to go, and the word stays perfectly readable without it.
Each spelling below has its own page, and every one of them leads to the same app. Pick whichever you searched for.