HC-SR04 / HY-SRF05 distance sensor not working on Pico?

I just did some (un)scientific testing and still can't get the thing to get the right distance. I moved the sensor the the very edge of the desk at the edge of the breadboard and measured some distances using a ruler and the handy Espruino reference card that I got in the kit and got the following values (actual measurement on left, console.log on right):

1. cm = 1cm
   
2. cm = 1.7cm
   
3. cm = 2cm
   
4. cm = 2.6cm
   
5. cm = 3.3cm
   
6. cm = 3.7cm
   
7. +cm = 4.1
   
   

I am guessing it's probably something to do with my module, but does that kind of pattern make any sense to you? Some kind of resistor needed? Again, apologies for my utter ignorance about this stuff.