Man, it seems that the lengths of the 'under consideration' period can vary by quite differing time frames. Some say 30 days from the extension applying date. Some say up to the current extension date if applying more than 30 days in advance of it. And your experience is an under consideration time frame lasting till 30 days after the current extension. And you were applying well in advance, that could mean a two-month time frame for the period. Hard to get a handle on how it should go, or how it's supposed to go. Just like almost all encounters with things immigration, it's a real case-by-case, 'your experience may vary' situation.
By the way, you said the under review period given to you always extends to 30 days after the end of your current extension's expiry date, but that it's usually approved before that date. So a question here. Do you return to immigration on that 30th day of the period, and discover at that time that the approval date was a month prior? Or are you able to get the finalized new extension before your current one has expired, before the end of the under consideration period? Just wondering how that works. If you apply 45 days in advance, and you are supposed to return on the 30th, the last day, of the under consideration period, correct? That would be about 75 days in a limbo of some type. If that is the case, of how that works.