Stamina and HP are the same thing???

Yes, poor word choice by Niantic

Stamina is a placeholder number because HP varies. Most of the time HP=Stamina, but while placed in a gym HP=2*Stamina.

Stamina is a fixed number depending on the pokemon species, e.g. Chansey has a Stamina of 500. The 15 points extra of the IV Stamina are added here.

HP are the total Stamina (Base + IV Sta) multiplied by the level constant (CP Multiplier). HP are shown in the pokemon profile.