Same as others have said Guinea pig food is higher in protein and lower in fibre. If we look at the excel pellet range... 5 % difference in protein is a lot.
supa excel guinea
Protein 17%
Oil 4%
Ash 6.5%
Fibre 15%
Calcium 0.8%
Phosphorous 0.5%
Vitamin A 22,000iu/kg
Vitamin D3 1,250iu/kg
Vitamin E * 100mg/kg
Vitamin C 800mg/kg
Copper ** 15mg/kg
rabbit
Protein 12%
Oil 4%
Ash 7.5%
Fibre 16%
Calcium 0.90%
Phosphorous 0.51%
Vitamin A 15,00iu/kg
Vitamin D3 1,500iu/kg
Vitamin E * 100mg/kg
Vitamin C 60mg/kg
Copper ** 15mg/kg
There are also different vitamin contents. If I remember rightly g-pigs can't make their own vitamin c so need more of that.