Why you don't count annual mileage exactly?

Because its impossible.

First, to review an exact 365-day period we'd have to require that you complete your odometer statement on a specific day, exactly 365 days after the first one.  Thats just not going to work in the real world.  Plus, we'd be required to nonrenew a policy if someone isn't able to comply on the exact day.

Next, there's the problem of a 'non-renewal window'.  If we find someone abusing their mileage limit, we have to non-renew the policy.  However, we are obligated to provide advance notice of this action (the exact timing is statutory and varies by state).  This allows the customer time to fix things if they simply goofed, or to place coverage elsewhere.

So even if we demanded an odometer reading exactly 365 days from your previous reading (and only on that day), we couldn't act on it.  The 'nonrenewal window' requirement wouldn't let us nonrenew someone ignoring the rules.  

We prefer not to lose customers, but we must be able to non-renew a policy to protect everybodys' low rates from those who are unable or unwilling to play by the rules.