Because it may be possible that you are either 1, 2 or 3 days late. If your journey starting station falls on 2nd day of the train, the booking opens 91 days in advance, if 3rd day, the booking opens 92 days in advance for that station. If the quota in which you are booking is small, it may not take long enough to get full, even if its the 1st day at your station.
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Also make sure you have correctly calculated. 90 days in advance is "excluding the date of journey".