The overall rating reflects the current state of the app. It accounts for all app reviews but prioritizes the most recent ones.
Do not use these people because they are dishonest and will try to blackmail you into leaving them a good review. I cancelled this app 5 days into the billing cycle and I was still charged for the entire month. When I asked for a refund I was told "Generally we don’t provide refund. But in your case I can provide you with a refund if you leave a nice 5 star review on our Shopify page". Shopify support isn't any help either. They bill you on behalf of the app company but either can't or won't get your money back from the app developers. I am forced now to dispute this with my credit card company because I will not be coerced into leaving a nice 5 star review if I don't want too.