Differences in bread texture

I've been making sourdough bread and challah for some time now. The two breads have different textures (challah is much softer) and I'm trying to understand why. The sourdough I make with 65% hydration, the challah is 60% not considering oil and 65% if I take the oil into account (as if it is water). 

Is the difference in the use of starter vs dry yeast? Use of oil? Anything else?

Put it another way, let's say I want to make a challah that is the same texture as my sourdough bread (that is, like plain bread). What is the change I need to make? 

Here's the challah recipe for reference:

Ingrediants:

A quarter cup (50 grams) of oil

2 and a half cups (600 ml) of lukewarm water

1 kg of sifted flour

A quarter cup (50 grams) of sugar

1 tablespoon dry yeast

1 tablespoon salt

Preparation: 

Mix the dry ingredients, add oil and water

Mix in a mixer on low speed until combined and then medium for about 6-7 minutes, until a ball of dough is formed.

Cover and wait for the dough to double

When the dough doubles in volume, divide it into 2 equal parts. Braid each part

Preheat oven to 220 degrees (or 200 in turbo).

Cover the shaped challahs with a towel and let it rise for about half an hour or until the volume almost doubles and the challahs round nicely.

Brush the challahs with beaten egg with salt and sugar, sprinkle with sesame seeds and bake for 25 minutes, or until the challahs are nicely baked and golden. Cool completely and serve.