you start a battle with a 5 chevron unit. and when you start the game the unit looses one chevron and goes down to 4. and after the battle it has five again, without gaining a chevron in the battle. now thats a bug, but i tought it was fixed in 1.2
Maybe your general has a valour penalty. It was certainly possible for generals to give valour bonuses in Shogun and Medieval - so that, say, a valour 2 unit on the strategic map appears as a valour 4 unit in battle. Havent noticed whether this happens in Rome, cos (a) changes in valour arent as obviously noticable with chevrons instead of numbers, and (b)the whole valour thing doesnt seem to make as much difference as it used to anyway.

But anyway, i guess its possible that your general has a trait that gives a -1 morale penalty to you troops during battle (though i dunno if there even are any such traits in the game, havent looked) so that your troops are v5 on the strategic map, but v4 in battle.