Well the answer goes to the context of GTX66's reply "got two updates within mintes of each other" as if that multiple updates in a short time frame might be a contributing factor. Since it can happen in the normal time frame of updates, usually hours apart then the frequency of the update isn't a factor.
Yes you will generally will get your first update shortly after you start your system as there is an update check very shortly after you boot, this will have been many hours since your last update so one is more likely to be available after this duration. But it will also depend on what time you boot and your time zone.
The one thing that struck me is the frequency of the update, personally unless you have the paid version/s of avast, then you shouldn't get two virus definitions auto updates within two minutes as the default update check is 4 hours with a minimum setting of 2 hours. Even with the paid version/s of the lowest update check frequency should be greater than a few minutes. But even for whatever reason you are able to receive two in such a short frequency I wouldn't be concerned, but happy to receive it/them.
That after all is the purpose of the streaming updates, to push out urgent updates that shouldn't wait for the next regular VPS update.
You are only living up to your user name ManyQs