Why is my iPhone 4S breaking up SMS messages that are shorter than 160 characters?

Question

With my iPhone 4S running 5.1, I'll tap out a message that reads on the character counter as less than 160 characters. When I press send, I hear the "sent message" swoosh sound happen twice, as it does when the phone splits a long message into multiple parts.
The recipient confirms that the message does get split into two parts even though its contents did not exceed 160 characters. Through some experimenting, I have determined that it happens with messages that exceed 120 characters. In cases of messages with lengths 120-160 characters, it cuts the last word off and sends it as a separate message. 
Why is this happening?
(In addition, checking my Google Voice history also confirms that the messages get split up even though they do not exceed the character limit.)

Answer 1

I'm having the same exact issue. I've only noticed it since I upgraded to IOS 5.1. Unless the SMS message is very short, it gets broken into at least two separate messages when I hit send. I think this is happening even if the message I'm sending is less than 120 characters.
It doesn't bother me either, but like AmZam05 mentions, I feel bad for the recipient who is now getting mulitple SMS messages from me when they would get a single SMS message prior to me upgrading to IOS 5.1.
EDIT: Wanted to add that this is with a Sprint iPhone 4S w/ IOS 5.1. I've also found some forum threads elsewhere speaking of the same problem others are having. There seems to be a "workaround" for this by adding a few blank spaces at the end of your SMS text message, then sending it.