Why the insistence on who has the most dice after removals?
Because the dice that are eliminated could eliminate possible results of the since every matching pair must be removed not just one dice from every player.

When they say all possible eliminations must be made, they mean that for each attacker/defender die elimination you can make, you must make that elimination. It is not trying to say that an attacker's own dice can eliminate each other, otherwise we wouldn't have this second sentence saying they can't do that

Beta Rules


  Fire and Water eliminate each other.
  Earth and Air eliminate each other.
  Spirit and Void eliminate each other.    
  
  When two dice eliminate each other, remove them from the pool of rolled dice. Dice that the attacker rolled can only eliminate dice that the defender rolled and vice versa. All possible eliminations must be made.
  
  Spirit: The attacker is Injured.
  Fire:​All ninjas in the attacker’s influence zone, including the attacker, are stunned. (See Stun, page !X!)
  Air:​The attacker may move the defender up to 3 spaces in any direction.
  Water:The player who picked Water as the result may move the attacker up to 3 spaces in any direction. The defending player then places the defender in the attacker’s front influence zone.
  Earth: ​The defender is stunned. (See Stun, page !X!)
  Void: ​The defender is Injured.     


Some examples:

Attacker rolls 2 Spirit and Defender rolls 2 Void


Check for matches
Find a match of 1 Spirit and 1 Void and remove
Check for more matches
Find a match of 1 Spirit and 1 Void and remove
No more dice so attack results in nothing happening    


Attacker rolls 3 Spirit and Defender rolls 2 Void


Check for matches
Find a match of 1 Spirit and 1 Void and remove
Check for more matches
Find a match of 1 Spirit and 1 Void and remove
Check for matches
Only 1 dice left and is Spirit so attacker is damaged


Attacker rolls 3 spirit, 1 Void and defender rolls 2 void


Check for matches
Find a match of 1 Spirit and 1 Void and remove 1 Spirit from attacker and 1 
Void from defender
Check for more matches
Find a match of 1 Spirit and 1 Void and remove remove 1 Spirit from attacker and 1 Void from defender
Check for matches
The 1 Spirit and 1 Void that are left are in the attackers pool so not a match and the attacker can now chose which effect to use