Maybe that five renewals is to cover the cases where pilots are known to log false instrument flying hours because they know there is no way these hours can be verified. Saw one airline first officer's log book where he had logged something like 2500 hours instrument flying in a total of 5000 hours.
Since single pilot actual in flight instrument flying can rarely be verified there seems little point in logging it in the first place. Decades ago, instrument flying hours in the log book was a measure of true IMC flying experience. And it had to be manually flown. But now that it is legal to sit back and log automatic pilot instrument flying hours that criteria for experience is not worth a pinch of salt.