Actually at the battle of hastings the Norman cavalry used throwing spears fighting from horseback with a spear is crap going over the horses head is awkward a sword is a far better option for that situation.
The overhand position as depicted in the Bayeux Tapestry is more commonly interpreted as simply an overhand thrust with the spear, rather than a throwing the spears -- although, no doubt some of the Normans did throw spears. However, at least, more depiction of what are clearly javelins, are associated with the Saxons (soldiers holding multiple spears). And of course, couching was in the battle.
(ps: ironically, the tapestry also depicts a fairly long spear easily matching the size of late-medieval ones, but this is usually viewed as the artist's creative choice)
As for time period i seriously wouldn't get too caught up on that we have counterweight trebuchets, coats of plate, stone castles, mail and plate horse barding to name just a few items that are from later dates.
IMO it comes down to the choice of which depictions matter more, and why such a choice is made. And in this case, the shorter lances IMO do make sense. The cavalry we have are not the Swadian Knights which you could line up and order a frontal charge with those long lances, and I think the intent of the devs (if there was an intent behind the choice) is for us players to recognize the couching method as an opportunistic method of attack, rather than the "ultimate charge weapon" it was back in Warband.
As it is, with the shorter cavalry spears, we are to rely more heavily on conventional thrusting with the lance, but when an opportunity arises, we can couch it for maximum effect. I wouldn't hesitate to couch the longest lances back in Warband, but in this game, I wouldn't try to couch the lance against spearmen that are already in position and aware of my approach.
I think this makes better sense with the overall feel of the game.