How does this happen?

Last month our electric bill was three times what it would normally be. There was no apparent reason for this since our habits had not changed. The only thing I thought it might be was the fact that we started using an electric blanket. I lectured everyone in the house to watch any unnecessary electric use and hoped for the best. The bill indicated that this was an "actual read" bill meaning that someone from the electric company read my meter to state we had used that much electric. This month I get two electric bills and one was a "rebill" for last month. It indicated an estimated usage for that month within our normal range of use. This month's bill was an actual bill and within normal limits. This just seems weird to me. I am glad that we didn't go crazy with our usage last month but can't help but wonder about the accuracy of these "actual read" bills.