Well they could all be singular just fine, but the first two were added a long time ago (and appear next to each other in the bonus list), whereas the third was added later on in the list. The better option is probably the plural approach, to better reflect that it is a cumulative stat, but not absolutely required which is why my mentality on adding the second was a bit different. Also the internal names for all of these are actually singular (the idea being that this is your bonus "per such kill," singular), so that was another influencing factor when I went to type out the public version of the stat.