AKILL was never designed with such short ban periods in mind.. if you want to ban someone for such a short period of time, you are better off using the IRCd's capabilities..
AKILL was designed for very long term bans, typically those exceeding the standard time of ircd level bans.. the reasoning was if you wanted to ban a 1000 masks, anope would check those after the client connected rather then the ircd checking them during the connection process as that would slow down every client connecting to the ircd.
This is also why anope places a time limited GLINE...
Assume you add an akill which will expire in 6 months: anope will place a GLINE valid for 48hrs or so and thus the client will be banned at ircd level for the first 2 days. after that, the GLINE is removed and thus connecting clients won't be slowed by it anymore. Anope checks all connecting clients against the AKILL list, meaning that if a client matching the akill connects, anope will detect it and place another 48hrs GLINE.. the result is that the client remains banned from the network, but the workload is moved away from the connection process in the ircd..
After 6 months, the akill will come to expire and since this is checked during the DB update process, it stays in the list until the next DB update.. in the very unlikely event there is still a GLINE in place, it will only be removed when the akill is actually purged from the list.
The impact of all this on the performance may not be that important anymore in today's world, but it was when this was conceived..