A package can be found in many repos, but it will always show and install the latest version. For example you can have power kernel v42, 46 and 48, depending what repos you enabled. If you only have extras, it will list v42, in extras-testing you will find v46, however if you have extras-devel, it will list v48. If you disable extras-devel, but have extras and extras-testing, it will list v46, even thou v42 is also present in extras. You can also specifically install a desired version by the apt-get install command, writing the full name and version of the package (like kernel-power=[version]).