howste, I find that this might be due to size and complexity of the organization. Most of my clients are quite small, and can easily audit the entire system in a day or less. To them it makes more sense to audit twice a year. One client I had was very large, and had a full-time audit staff. They audited everyday, across 3 shifts. They still had a hard time auditing the entire system in a year, not that any auditor would goldbrick or anything.
If, based on your size, complexity and resources available, you might want to try one way. If it doesn't work, change it.