@Mark is correct, it's not a bug.

In the first example:

It tries to match each alternative at position 0. Failure.
It tries to match each alternative at position 1. Failure.
It tries to match each alternative at position 2. Failure.
It tries to match each alternative at position 3. Success. ' inc' matches.

In the second example:

It tries to match each alternative at position 0. Failure.
It tries to match each alternative at position 1. Failure.
It tries to match each alternative at position 2. Failure.
It tries to match each alternative at position 3. Failure.
It tries to match each alternative at position 4. Success. 'incorporated' matches. ('inc' is a later alternative; it's considered only if the earlier alternatives have failed to match at that position.)