How come two sister companies can have different quality of services.

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My old telecom which is Smart and my current Telecom which is PLDT are sister companies. Pldt is not perfect but I love the service they provided me these days because they don't have a date cap. They forced to have a Landline phone line because it is part of my Internet package but their service is tolerable. They don't have data cap and they gave me what they promised. I asked for a 10mbps internet connection and that is what they gave me. Smartbro on the other hand gave me nothing but stress with their garbage connection. I checked out their facebook page and wow they were still bombarded with complaints. This company is making billions of dollars with their garbage service. I find it weird that these 2 companies are from the same corporation.