`sh` is only supposed to be _a_ POSIX-compliant shell, it's not odd/interesting/confusing that Ubuntu uses dash any more than it is that something else uses bash.

The shebang line #!/bin/sh should only be used for POSIX-compliant scripts; if it needs {bash, dash, python, etc.} it should start #!/usr/bin/env {bash, dash, python, etc.}.

In fairness, that's just for sh; if for some reason you actually need bash you can just use `#!/usr/bin/env bash`