Why is it that the Grand Exchange price doesn't always adjust to OSRS Gold meet demand and supply? Other examples could be provided which will show how Grand Exchange prices don't appear to change in response to current supply/demand.
Gielinor's technology level is predominately about the 15th or 16th century (a period that is admittedly more lengthy than most RPGs that are fantasy) However, with Renaissance garb, cannon-topped castles, and cannon-topped castles, there are some striking historical relics.
A lot of homes and structures are equipped with water-tight sinks. This advance was possible due to the invention of running water in many locations. It could, however, have been orchestrated by the power of magic, however it would still have an enormous effect on the entire society.
Others are easy to spot - one the prime offenders in this area is the really sort of silly modernity of machines like the ones found in the Elemental Workshop (these, of course, are powered by magic, however their designs are totally incompatible with the other world).
The vast variety of names from the real world that are found in Gielinor isn't as tangible, but it's real. If Gielinorian humans were actually from Earth, a theory which I don't like but that is possible, Gielinor could have survived vestiges of "ancient" names, but they'd probably be Cheap RuneScape Gold drastically modified and would not be in widespread use.