Character Count in Messages Not Always Correct

Has anyone out there who has enabled the character count in Messages experienced the following. It is my understanding that 160 characters in the maximum for a single text message. In other words if you type more than 160 characters, then the text is now counted as 2 text messages, over 320 characters is counted as 3 texts, etc., etc. Well, on my phone sometimes my first text to someone shows the counter as so many characters out of 70, not 160. The counter may look something like this: 38/70. Usually on my second text message to the same recipient the counter returns to the number of characters slash 160. Anyone know why this is happening? It doesn’t do it all the time. It happened once yesterday, but now this morning my first text to someone was /160 like it is supposed to be. Thanks for your insight!