There are many logical explanations for this. Here's the most important note about speakers I can pass on to you. That would be best compared to having a 2 inch water pipe coming from your street to your house, having a 1 inch water pipe running to all the faucets in your house and connecting the two by reducing each of them to a half inch connector. This translates into a loss of power. Everyone in the room could hear the difference in volume between the two speakers.
All speakers have an impedance measurement just as they have a diameter measurement.
Not all speakers are 15 inches in diameter and not all speakers are 8 ohms. If you have two of them in one cabinet you can wire the two woofers in parallel to create a 4 ohm cabinet, or you can wire them in series to create a 16 ohm cabinet. Since the 16 ohm cabinet will need more power to produce volume, many lower end companies mass produce these cabinets and heavily advertise the high power that they can handle, as if that makes it a better speaker than the more efficient and lower power speaker. This is the most misleading specification we see in advertising. One club audio company that advertises as much or more than anyone else makes a dual 15 cabinet with two 16 ohm woofers wired parallel to create an 8 ohm cabinet.