cold and hot line, then cold water could be mixing with the recirculating hot water causing the whole house to be lukewarm. A recirc pump is installed in a loop, so that hot water is always recirculating around the house, ready to immediately arrive in your shower or sink when you call for hot water. Rather than waiting for it to make the trip from your water heater, through cool pipes, to your shower.

If your home is relatively new, that may be a feature that was installed when built. You'd likely see a small pump plumbed into some pipes above the water heater. If that loop had cold water introduced to it somewhere beyond the water heater, that could cause the whole loop to be lukewarm, and every faucet would get lukewarm water when you called for hot.

That's all I can think of that MAY cause the issue to affect the whole house. Otherwise, if the water is hot leaving the heater, I don't know what would cause it to not be hot throughout the house at every fixture. That being said, I'm not even sure crossing the lines would do it either, but that's all I've got.