The fact that it can detect more APs does not mean it can transmit at a higher power. What it on the other hand does indicate is that it is more sensitive than your external card, which I find rather curious.
Please do also remember that the PWR ratings given by for example airodump-ng can NOT be used to compare two different cards. It should only be used to compare different APs using the same card and same drivers. The difference between the two cards might therefore not be as extreme as the PWR ratings can seem to suggest.