IE cache affecting the karma coverage

I found that something wrong with my karma coverage.

Here is my preprocessors and browser configuration:
preprocessors: {
'lib/src/src1.js' : ['coverage'],
'lib/src/src2.js' : ['coverage']
},
browsers: ['IE', 'Chrome'],

Firstly, reports in both IE and Chrome show the coverage correctly. Then I remove 'lib/src/src1.js' in my preprocessors's configuration. This time, report of Chrome shows only about src1.js's coverage. However, IE report still shows the coverage of both src1.js and src2.js.

I was wondering that if this is affected by the IE cache. So I clear the IE cache thoroughly. Magically, reports in IE and Chrome are correct again.

How can I fix this issue ??? Is there any wrong with my configurations ??? Or is there any version of karma-coverage fix this bug???

ps. I had tested in both IE10 and IE11, windows 7. It comes the same result.

Many thx!!