Cause: Strider has a turret while the Irfit and Hunter used by the other factions don't have turrets. Therefore you get a group of 5 units not 4 as intended when using BIS_fnc_spawnGroup:

several functions are called with BIS_fnc_spawnGroup:

BIS_fnc_spawnVehicle
BIS_fnc_spawnCrew.sqf
//Search through all turrets and spawn crew for these as well.<=== extra crew member spawned as it searches turrets

> BIS_fnc_returnVehicleTurrets;

_grpName = [(player modelToWorld [0,20,0]), resistance, (configfile >> "CfgGroups" >> "Indep" >> "IND_F" >> "Motorized" >> "HAF_MotInf_GMGTeam")] call BIS_fnc_spawnGroup;

Actual Behaviour - see config entry below:
unit0 "I_soldier_F" - Driver
unit0b "I_soldier_F" <======spawns extra unit in commander turret
unit1 "I_Soldier_TL_F" - on foot
unit2 "I_support_GMG_F" - on foot
unit3 "I_support_AMG_F" - ****no room in vehicle for this guy

Temporary fix - delete the extra soldier in the turret or the driver?:
_vehicleName = vehicle (leader _groupName);
_unitName = _vehicleName turretUnit [0]; moveOut _unitName; deleteVehicle _unitName;

Possible Solution:
So you need to delete the Team Leader from the cfgGroups entry to get the right number as there is already a commander in the vehicle?:

~~~~~~~~~~~~~~~~~~~~~~~~~
class CfgGroups {
~~~~~~~~~~~~~~~~~~~~~~~~
class Indep {
name = "Independent";
~~~~~~~~~~~~~~~~~~~~~~~~
class Motorized {
~~~~~~~~~~~~~~~~~~~~~~~~
class HAF_MotInf_GMGTeam
{
name = "Motorized GMG Team";
side = 2;
faction = "IND_F";

class Unit0 {
side = 2;
vehicle = "I_MRAP_03_F";
rank = "SERGEANT";
position[] = {0,0,0};
};
class Unit1 { <============ Delete this entry so enough seats for team - the commander that spawns in the vehicle is assigned as group leader by default.
side = 2;

	 vehicle = "I_soldier_TL_F";

rank = "SERGEANT";

	 position[] = {5,0,0};

};
class Unit2 {
side = 2;

 	vehicle = "I_support_GMG_F";
	 rank = "CORPORAL";
 	position[] = {-5,-5,0};

};
class Unit3 {
side = 2;
vehicle = "I_support_AMG_F";
rank = "PRIVATE";
position[] = {5,-5,0};
};

You would need to do the same or check all Independent motorized classes that use the "I_MRAP_03_F" Strider:
"HAF_MotInf_Team" <=== OK 3 units
"HAF_MotInf_AT" <=== ! 4 units
"HAF_MotInf_AA" <=== ! 4 units
"OIA_MotInf_GMGTeam" <=== ! 4 units
"OIA_MotInf_MGTeam" <=== ! 4 units
"OIA_MotInf_MortTeam" <=== ! 4 units