We can't really support mapping by A Record. We do not have static IP addresses, which makes pointing to an A record problematic at best.
To explain briefly, we have a growing number of datacenters with thousands of servers between them. WordPress.com is already built on top of a cloud architecture, so when one server goes offline, everything is routed over to another transparently. When your domain is mapped to us by name servers, this all happens automatically.
With an A Record, you're directing the domain to one IP. If that server/IP goes offline, so does your site, there's nothing we can do about that.
There's a bit more involved, but that's the short version.
If you want consistent performance with a mapped domain on WordPress.com, you'll need to direct the name servers to us.