@HenryThePear After watching the 2 videos, you score much higher, because 2 things:
In the First Video you would plant food the Primal Mines first instead of the Imp Pear.
Thanks for this as this is a tip I need to share on my videos and that is you have to PF the Imp Pear first. This way when PPM land they do not get eaten by the higher level Bucket and Blockheads, before they arm.
So, this answers @PowerToss question, that Mines that are eaten before fully armed DO lower the ZDL. (This has been confirmed by Haem on Reddit).
The 2nd thing, in the first video you held on to your Plant Food. In the second you didn't. The quickest way to raise the ZDL is to kill the first waves as quick as possible. ZDL isn't only affected by plant deaths, it looks at how long it took you to clear most of the previous wave. It determines if you are struggling and "Helps" you by producing lower health (and lower scoring) Zombies. This works conversely, If you had no problem clearing the previous wave, the next will be Zombies with Higher Health or just more of them (Higher ZDL).
Hopefully this helps.