Rebecca,  
   Your question is an interesting one, but one that is going to be next to impossible for me to give you a precise answer.  First off, I'm going to have to make a few assumptions regarding some items you didn't mention in your question. 
  (1) Variety; I must assume all the hydrangeas you planted were the same variety. For example, we have a number of hydrangeas in our yard, but one is a climbing hydrangea which we planted at the base of a Red Maple tree. It tended to grow more laterally until it began to climb. In addition to the climbing hydrangea, there are also a few hydrangeas that have a vining or ground cover-like habit. The one you have that is failing to grow vertically may be a climbing or vining variety that somehow got mixed in with the other hydrangeas at the growers or at the garden center or may have just been mislabeled.
(2) Light. I must assume all were planted in similar light situations. For example; my hydrangeas are all in partial shade and the sun exposure they get is morning sun up until about noon. If this one hydrangea were to get afternoon sun, while the others got morning sun, that could make this one plant slower to take hold and effect its growth habit; especially with the hot summer we had this year.
(3) Soil. I assume all had similar soil conditions. I have seen incidents where one plant failed to grow as well or keep up with others planted at the same time. We found out later there was a large rock just a little lower under the soil where the one was planted and it stunted the growth of the plant above it.
(4) Moisture/Drainage. Again I have to assume all the hydrangeas have similar exposure to moisture and the drainage conditions were alike.  Being planted on a small mound where it tends to be dryer or in a bowl like depression where it tends to stay damp more than the others can affect it's growth.
  Having said all the above, let's assume there was no mix up in labeling, all the varieties are the same and the growing conditions are at least closely similar. In that case I begin to suspect there could have been some root damage occur or this one plant happened to be a little more root bound prior to planting than the others. Since you say the plant is otherwise healthy, flowered and is rounded and showing consistent growth, I would recommend you keep treating it just as you do the others and see if, in the future, it doesn't catch up with them.
  Lastly, if this one plant remains stunted in it's growth when compared to the others, then I suspect one of the other variables mentioned above is coming into play. However, as long as the hydrangea is health and thriving, the fact that it is not as tall as the others, really shouldn't be a item to worry about unless you're using them as a hedge and the one just looks out of place.
  Sorry, I couldn't tell you exactly why your one hydrangea wasn't keeping up with the others you planted, but I, hopefully, gave you some things to consider if you find the situation continues to persist. Have a great fall season and I hope you enjoy the cooler weather (when it gets here).