Every single individual Pokemon has a predetermined, unchangeable value known as individual values, or IVs, for each one of the six stats. These range on a scale of 0-31. The higher your IVs are in a stat, the higher your stats will be in said stat. Likely, your Platinum Monferno has a lower Sp. Def or HP IV than your Diamond/Pearl Monferno.
In short, yes, it's just you.