Battery drainage in custom watchfaces

What's the reason all (at least those I tried) custom watchfaces use more battery than the stock ones?

E.g. stock Fenix 6 watchface let the watch for for ~13 days as for my usage but Phoenix V2 lasts for max 10-11 days and that's quite good. 

2nd E.g. stock Vivoactive 4 watchface lasts for 7-8 days, custom Fusion lasts for 5-6 days.

At the moment I have Phoenix V2 and the following data fields:

1. the ones I use also in the stock watchface

- steps

- floors

- battery

- temperature (weather)

- calories

- date

2. additional ones

- distance

- air pressure

- altitude

Is it a matter of particular data fields or the differences in custom watchface implementation in comparison to the stock ones?