sed '10,15 <command>'
means do the command over the range 10-15. In you case, the command is "i" for insert. Thus you wind up with the inserted text on every other line. (Specifically, "i" inserts a new line in place of the one specified--and then moves everything down.)
In my docs, I don't see how to do what you want without simply having two commands, eg:
sed -e '10 i <text>' -e '15 i <text>' filename
The addresses and <text> can both be variables. (Use double-quotes if you have a "$" in the sed expression---or escape it.)
Good SED tutorial here: