Wrong estimated number of rows using TOP condition
13.83 is the number of rows it thinks it needs to satisfy the 15 rows you're asking for. It guesses that after that it can stop.

Paul White goes into it a lot more in his post Inside Optimizer Row Goals in Depth. 

Since he spends a lot of time at this site you'll probably see an extensive answer, going into a lot of detail about why it's 13.83 and not 13.84. ;)

I suggest you to read below blogs


Why Top1 Killed My Performance you'll also see some reasons why having row goals can hurt.  
SSIS Tuning tips That Everyone Misses