Large Security companies merge
Apollo now owns ADT and Protection one so essentially they are the same company but they are currently still operating as two different companies. Always check the better business bureau on the current and the older companies names before you purchase.
TYCO and Johnson Controls recently merged. Often the engineering specifications will list one of these installing companies as a brand name rather than an installer and the engineers list them as manufacturers. TYCO does manufacture products but those are separate from this business as far as I know.
Tyco and ADT were together and now that they are separate
Normally these companies purchase OEM products and put their name on them. Some other examples of Companies buying other companies:
Bosch bought Phillips and they also bought Detection Systems and Radionics several years ago.