Are you worried about it? The systolic seems kind of low, but if she did it manually (vs using a machine) it could have been human error - she didn't hear the first beat or something. However, dehydration will cause a low BP, and you mentioned two posts ago that you're dehydrated.
According to Mayo though, anything under 120 is normal. So I wouldn't worry about it. Besides, 3 seconds later she could have gotten a 115/78 reading. BP fluctuates constantly.