The question I have to ask is are you looking at the stats with enchanted items on or off? Basic damage will vary depending on the gear worn with enchantments. If I have 4 items with archery enchantments equipped after smithing a bow then the stats will be a lot higher than without wearing them. A bow with 155 basic damage (legendary) can jump to 355 or greater depending on the enchanting applied to the gear.
Take off all your armour and ring and check the damage, you may have been wearing some archery modifier when you first improved the bow. If you know the game and how enchanting works on gear then ignore all I've said, it must be one of skyrims famous glitches.