As they both have shields and as shields are extremely effective against missile fire (double parry) it doesn't surprise me the least. Of course, armor is also good when facing archers and the like but it is not so price worthy as shields.
I use City Guards as cheap and effective early expanders.
Here are some pros that make them good at this:
+ Shields for facing archers
+ Pretty good armor for facing milita, light infantry and other spear wielding units
+ Reasonable encumbrance for fighting high defense units like the above as they don't have very good attack skill and only low damage spear.
+ Moderate AP. They can get "first hit" if you script them right. They can also chase and backstab most routers (-4 def) and get valuable experience.
+ Moderate defense skill (12). Will have a pretty good chance to avoid getting hit by light infantry (att 10), and tribe warriors.
+ High map move (2) for armored unit. You can return back to your castle to gather reinforcements or for defense.
+ Fortress defense bonus. When you are done expanding they don't become useless as you can put them in your newly built castles for defense; they will also gather experience over time becoming more useful again.
+ Good morale for lizard unit. VERY good in the beginning when you don't have dominion over a province (+1 to your moral -1 to enemies) you want to conquer from independences (they get +1 to their morale as it is their home province).
Cons
- No helmets. Very prone to getting critical once fatigued so use a small screen of light infantry with shields to draw arrow fire away from them.
- No missile units for "softening up" advancing infantry for killing with their low damage spears.
Two ways to do this:
*Fire closest/no scripting put forward
+ Will fire into the enemy ranks
+ Will build up fatigue (very effective when facing heavily armored infantry) letting the City Guard get critical hits easily.
+ Will hold up pretty good against low attack low damage weapons as spears as they have 13 in defense.
+ Will draw some fire into the enemy rear from its own archers when in battle.
- Some of them will die of course but they are cheap so use many.
*Forward and hold attack
+ Will get fired at by archers even when the city guards runs past them.
+ Will not get harmed in close combat.
- Will fire into the battle between the city guards and the advancing enemy with their javelins.
There are lots more to this of course.
Sorry for being somewhat off-topic.