Why does the in-game delta-v counter show more delta-v for the Terrier compared to the Poodle?

[DoctorSnakes]

Despite the Poodle having more ISP in a vacuum, the Terrier has more delta-v (in a vacuum) in the screenshots in the spoiler:

Spoiler

The only mods that I use have nothing to do with changing how the rocket engines work or their delta-v, so why?

[James Kerman]

2 hours ago, DoctorSnakes said:

Despite the Poodle having more ISP in a vacuum, the Terrier has more delta-v (in a vacuum)

This is because the terrier has a much lower mass compared to the poodle which affects the dry weight of your rocket.

[king of nowhere]

when i was inexperienced, i too was ignoring engine mass and looking only at Isp. huge mistake. in ksp, engine mass is easily 20-30% of your total dry mass; it's a fairly large amount.

In your case, your fuel tank has 1 ton of dry weight, so the poodle engine would almost double the mass compared to the terrier+tank ensemble.

on the other hand, scrimping too much on engine mass will reduce thrust, which will make manuever more inefficient, leading to more deltaV losses than you gain.