Obviously this is just tolerance, honestly I kind of doubt that gels and powders do work the same and that just just have no tolerance to these, although I can believe maybe they work better still. This being the case, the tolerance you're experiencing probably has a significant absorption rate component which affects the biomechanics of the overall experience - thus leading you to experience a subjectively greater tolerance from syrup rather than other forms.
But, yeah - this is tolerance - there is no other explanation. Please do your best not to use DXM so often, I'm sure you already know it's not good for you. I'm not sure what PEG refers to but I would strongly advise against chasing a more powerful experience to a drug you are probably heavily tolerant to by adding another drug to the mix. Brain studies of habitual dissociative users typically show far more pronounced changes, even damage, when other substances are involved (stimulants especially, IIRC, but not exclusively).