Disciplining stepchildren

[SP]

I have had little trouble with SS6 himself in the 3 years I have lived with him. He is a good kid, he pretty much always does what I say with a minimum of fuss. To me, he seems to be a people pleaser- always looking for people to tell him how good he is- and part of his good behaviour I think is fueled by his desire for approval.

But I got a bit of a reality check the other day! I have noticed that SS will defer to me if his father and I are saying conflicting things. DH asked him the other day why, (SS didn't know I could hear) and SS said that it's because he's scared of me!

I was very taken aback. I am not the sort of person that shouts, or acts in an intimidating fashion, so I don't know where this is coming from. DH has always firmly held to the idea that SS should respect me the same as a real parent, and that SS is to listen to what I tell him. The only thing I can think of is this: as a discipliner, I am much more consistent than DH. If I tell SS he must do something, I watch him to make sure he does it, and he knows so. DH, he knows, as all kids learn which are the softer targets, is not quite as consistent and might forgot or not follow it up. So SS tests the boundaries with him much more.
But on the other hand, DH does at times lose his temper with SS in ways that I never would too!
So I am thinking SS is using the word scared, to indicate he knows I am strict and will make sure he's doing the right thing, where others might let things slide.

TBH, I am a bit worried SS might go and tell BM that he is scared of me- but I don't know how to fix the situation either! I don't lose my temper with SS, I don't shout at him, I very very rarely physically discipline him (which BM and SF do all the time), and I'm actually quite patient. The worst I might do when SS gets on my nerves is use a serious tone to show him that I am serious when he is in trouble/ has to do something.

So I don't really know what I can do about it? SS and I spend plenty of time doing fun things, so it's not like I'm this big scary figure in the background all the time. Nor do I think I should become slack with discipline just so SS doesn't think this way.

What would you do?