Weird gap in Defender Normal's difficulty curve

[Not-A-Normal-Robot]

So I recently graphed the difficulty curve of Defender Normal, and there's a weird jump in it on wave 120. This doesn't happen on Defender Lunatic, oddly.

Software I used for graphing: https://www.desmos.com/calculator
Equation I used for Defender Normal: f\left(x\right)=\left\{x\le60\ :\ 360-3x,\ \ \ \ \ x\le120\ :\ 300-2x,\ \ \ \ \ x\le180\ :\ 240-x,\ 60\right\}
Equation I used for Defender Lunatic: f\left(x\right)=\left\{x\le60\ :\ 240-2x,\ \ \ \ \ x\le120\ :\ 180-x,\ \ \ \ \ x\le180\ :\ 120-0.5x,\ \ \ \ \ 30\right\}