Archers increasing strengh?!

Hi

I'm playing RoM and I have just notice that the archer's strength increases over time.
When I started the game, they had strength 3 (as they should, as usual and as i says in the Civiclopedia).

But after, let say, 100 or 200 turns, I realize they now have strength 5, which is pretty overpowered. With strength 5, there is no point to build lancers(4) or Axemen (5).

I don't know if this is a bug or it is intended that unit's strength grows over time.

Thanks in advance.

It's intended and described in civilopedia, some techs give strength to some units.

Archers are supposed to carry the burden of city defense throughout the Ancient and Classical Eras, so they need to get better over time. Archers top out at 5 Strength (3 base, +1 from Warfare, +1 from Mathematics), when by about the same time, Light Swordsmen and Horse Archers (both Strength 7) are available. The AND unit strength curve is steeper than BTS units (compare BTS Modern Armor at 40 and AND Modern Armor at 105) so it's a lot harder for old units to stay relevant without some kind of upgrade, but we don't want to make too many units.