Maybe you already knew this, but time trial bikes are not only used in time trials, they are also used in the triathlon races. In this discipline the athlete first has to swim 3.8km than he has to ride 180km on a bicycle and to finish they have to run a marathon which is 42.195km. For the 180km cycling the athletes make use of a time trial bike. As you may have noticed the distance is much longer than a normal time trial, that’s why a triathlon bicycle is a bit different, it improves the comfort for the long distance.
The biggest difference with time trial is that the rules of the UCI don’t apply in triathlon; they follow a different set of rules created by the ITU (International Triathlon Union). These rules are less strict than the rules of the UCI which gives the bike manufacturers more designing freedom. Although there are some rumors that they are going to implement the rules of the UCI in the future.
Some manufacturers have created a special triathlon bike, like for example the shiv from specialized. This bike has a drinking reservoir in the frame of the bike which isn’t allowed by the UCI but can be used in triathlons. Other manufacturers don’t have a special triathlon bike but sell their time trial bikes with extras. Like for example extra bottle holders for extra drinking bottles, etc.