Dear Tom,
Unfortunately, SAM's tiered rate model does not accumulate kWh on a monthly basis, so it does not correctly model tiered rates. It treats the tiers as hourly tiers instead of monthly tiers.
In your file, for example, the Tier 3 rate of $0.32377/kWh will only apply in hours that the hourly usage exceeds 30.6 kWh.
We plan to fix this behavior in the next version of SAM, which we hope to release in the Fall of 2014.
Best regards,
Paul.