version 10 - Global context seems to be shared between kernels when using Clear?

I am trying to understand the following behaviour, and determine if it's a bug in Mathematica 10. I am using Mathematica 10.3, and can't reproduce this behaviour under Mathematica 9. The test outlined below shows that evaluating Clear["Global`*"] under one kernel clears the Global context in another kernel, which I don't understand - I thought each kernel had it's own Global context.

To demonstrate, I have created a simple notebook called clear.nb containing:

Clear["Global`*"]

and nothing else. I have also created two new kernels called Test1 and Test2.

Now in another notebook I have the following:

f = 99.0

nb = NotebookOpen["/path/to/clear.nb"]

SetOptions[nb, Evaluator -> "Test1"]


NotebookEvaluate[nb]

Evaluating this notebook under kernel Test2, I find that immediately after the NotebookEvaluate[nb] both nb and f are no longer defined. In other words the Clear["Global`*"] which has been evaluated by kernel Test1 has cleared the Global context of kernel Test2.

This is not what I expected at all, and now how it worked in Mathematica 9, where the Global context of kernel Test2 is unaffected.

Is this a bug in Mathematica 10, or a bug in my understanding/expectation?