Why can't we test the pH of Oil Cleansers?

I know it's kind of "known" that you can't test pH of oil cleansers at home.  Like, there are industrial tool-mabobs that can test pH, but not something you can just snag on Amazon for fun.  But...is it true?

I'm not positing I have a concrete answer.  Far from it, in fact.  I am just experimenting and posting my own thinky thoughts, hoping someone will chime in.

This is my tap water, which is roughly 7ish (neutral).  I test my foaming cleansers either straight out of the container or if it's too thick and messy, lathered up with some water.  So what I did was I took my three snazzy balm cleansers...

And emulsified them with the tiniest bit of water.  Which is the same thing I do when I wash my face anyway.  Then when they're all nice and emulsified, I tested pH.  My results?

Banila Co, ~5ish.

The Face Shop, ~5ish again.

su:m37, ~5ish once more!
Is this bad science?  My logic is well, this is what I do to my oil cleansers anyway.  I put them on my face, and then I add water and emulsify.  The emulsion is still sitting on my face, its pH is still affecting my skin, isn't it?  I don't think it's a concern, because typically the only "problem" products are foaming cleansers, but it was still a curiosity for me.