The 13th month for requalifying is just EK's way of ensuring the next membership period is kept short and does not end up being at least a month longer each time due to the mismatch of the 13 month initial qualification period and the 12 month requalification cycle ;-)

Assuming I didn't get it wrong, the "generosity" works to their benefit, not the member's. The card is valid for 2 months beyond the 12 month requalification period, and their initial qualification period is assessed on a 13 month rolling basis. So assume you did not requalify within 12 months, but only had enough miles after 13 months: you would technically qualify as a "new" (so not requalify) Platinum, but this time round the next expiry date would be one month later. You would also not have lost out on any benefits in the interim, given that status is valid 2 months beyond expiry of the regular requalification period (given which, depending on your flight pattern and the rolling 13 month qualification period, you may even manage to qualify during the second month after expiry of the regular qualification period without loosing any of the benefits in the interim).