Hi,
This is documented in maintenance/tables.sql - check the comments at the top, it has something about timestamp fields.
Generally someone ages ago didn't like that TIMESTAMPs are auto- updated in MySQL and chose CHAR(14) instead of DATETIME. It would be pretty good idea to switch some day to datetime, and we have nearly all code in there. The sole problem is tables with tens of millions of rows to change. But that is doable, I suppose.
And in case anyone needs a cast, one can do that with say... ourfield +interval 0 second ;-)
Best regards,