This answer cites a source which explains that Kaddish for a parent is customarily not said for 12 months so as not to imply a parent's status as that of "wicked."
Not wanting to label a parent wicked might justify not filling out the full twelve month period, but it does not explain why, specifically, we stop at 11 months. Is there something independently significant about 11 months that makes it an appropriate minimum time for which to say Kaddish? Couldn't we say it for 11 months and 29 days (or any other time under 12 months)?
Is the soul of the "not wicked" judged for only 11 months giving that time relevance? If the soul is "under divine judgment" for the full time, wouldn't it benefit the soul for us to continue saying Kaddish until right before the label of "wicked" would be applied -- 12 months?