Consistently used phrase - interpretation?

[JackBlacknBlue]

In moment of weakness this weekend, I re-read some cards that my upBPD friend had given me over last several years. I am curious about the wording of her messages in the cards as I always found it strange that she worded the messages she wrote in 5-6 cards over the course of 4 years with two very similar variations on same message: 

"Thank you for... . (insert whatever I had done to help her).  You don't know how much it has meant."

"Thank you for... . (insert whatever I had done to help her).  I can never tell you how much it means."

When I thank someone, I usually tell them that whatever they did meant a lot or why it was meaningful.  I had always thought it was curious that she was vague about it, like it was an out for her to later say - "it didn't mean anything" because that wouldn't contradict the messages she wrote in the cards. 

Thoughts?