It appears that Disney takes hotel rooms seasonally in and out of service primarily to reduce costs, not to manipulate the occupancy rate, at least not the occupancy rate reported in Disney's financial disclosures.
Typically, a room taken out of service for a relatively short period of time has little effect on occupancy calculations. In Disney's case, rooms being renovated or converted to DVC may noticeably affect available room nights reported in its financial disclosures, but seasonal adjustments may not. For example, available room nights declined when rooms at the Polynesian or Wilderness Lodge were converted to DVC.
Let's consider what Disney has reported for available room nights since the start of fiscal year 2013, using the first fiscal quarter of 2013 as a baseline:
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With one exception, the available room nights varies by no more that +/- 1.5%. The one exception is the quarter when Disney included an extra business week, something it has to do every few years since the 365-day Leap Year calendar does not line up exactly with the 7-day week.
A common reason to take rooms out of service is to reduce operational costs. The room does not need to be cleaned, air conditioned, etc. Knowing how focused Disney is on margins, it's not at all surprising that Disney seasonally takes rooms out of service to squeeze a few percent out of hotel margins. This
can affect the rooms available for you and I to book However, it generally does not affect the occupancy rate Disney reports in its financial statements.