From the csplit man page:
Each PATTERN may be:
copy up to but not including specified line number
repeat the previous pattern specified number of times
Your command copy up to (but not including) the 10th line and repeat this three times. So your final result is 1 file of 9 lines, 3 files of 10 lines, 1 file with the rest. I don't know why the three files have 10 lines instead of 9, but the result is quite consistent with the command you used. What is the result you want to achieve? Maybe split -l
is more useful.