Why compare results in the chicken/fish example?

Suppose someone claims:

Desi chickens are more active and taste different from commercial poultry.

You need to determine whether the difference is really caused by the type of chicken or by something else.

So researchers compare:

breed — are genetically different breeds being compared?

feeding — did one receive different feed?

environment — was one allowed to roam?

housing — cage versus open environment?

age — older versus younger birds?

methodology — how was activity or taste measured?

sample size — were 5 birds compared or 500?

statistical analysis — could the difference simply be random variation?

For fish, you might similarly compare:

species,

water quality,

diet,

growth period,

stocking density,

age,

exercise/swimming conditions.

This is why saying “I observe a difference” is different from saying “I have demonstrated what causes the difference.”