This tester will run multiple HTTP GET calls from locations near CloudFlare PoPs to your destination URL.
Based on CloudFlare headers it will show whether the asset located on that URL is cached on particular PoP (HIT), or not (MISS).
Majority of CloudFlare customers do not have access to all the CloudFlare PoPs, testing on all the PoPs is on our roadmap.
If you observe which CloudFlare PoPs are being used in some loctions, you will notice that for differnet locations the same PoP is used. In such a case 1st request to the PoP will have a MISS but next requests which can come milliseconds after will already have a HIT.
See explanation here on the CloudFlare site.