Since WaniKani isn’t intended to teach us English, it’s set up to be a bit lenient when it comes to the English answers. Trouble is, as you’ve noticed, sometimes answers that are absolutely wrong are close enough to the correct answer to also be marked correct.
The way around this is to install a user script - specifically, the Double Check script. It allows you to mark correct answers as wrong. Alternately, you can set it to “close but no cigar” mode, which means it’ll no longer accept answers that aren’t precisely correct.