Some notes like "C#" or "Db", are actually the same tone. Only the name differs. This is called an enharmonic equivalent. "C#" is the enharmonic equivalent of "Db", meaning it is the same tone with a different name.
By default, the app is using the tones with fewer accidentals for better readability. If you want to switch to the enharmonic equivalent, you can do this by tapping the icon in the middle.
Note that this is not always possible. It is possible for (Ionian) keys:
Ab (G#) – A flat to G sharp
Db (C#) – D flat to C sharp
F# (Gb) – F sharp to G flat