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Coincidence? Some weird interaction between Kaspersky and your browser? I can't explain what's happening on your computer without seeing your computer.

My reasoning is that Speedtest.net does it's speed tests in a flash app, not in the browser, and it does not do file transfers through the browser, so in both respects the browser does not do the data retrieval (though it does do the socket handling) so shouldn't be caching it.

The speed tester itself also has built in "cache-prevention" mechanisms that explicitly preventing browsers caching the data. However they do admit " In rare circumstances some security software can act as a proxy, delivering the speed test payload from cache, meaning from the local memory on the client machine"

So:
a) The web browser isn't processing the data and shouldn't cache it
b) Browsers that do process it are specifically told not to cache it
c) Browsers that ignore that are still subject to "cache prevention" measures, whatever they are
d) Antivirus/firewall/filtering programs may ignore this still

As for Kaspersky we know it does d) but in relation to your tests, Kaspersky may clear it's own cache when you clear your browser cache - this would make sense - but this is just a random guess as to an explanation.

It's still the one (known) exception though, without Kaspersky browser cache should have no effect on speedtest.net results. If you have Kaspersky, exiting Kaspersky will more reliably fix the bogus results than clearing your browser cache. Cache clearing may have worked for you, but never did for me as long as I left Kaspersky on. You still don't need to clear the cache.