Indio, I am not sure if you can make conclusions on the relative strength of PS and RF eras based on evidence that you presented.
If top players in the 1990 s were more prone to lose in the early rounds of tournaments as compared to top players ten years later, then three kinds of effects may be mentioned to explain such a pattern.
- top players from the 1990 s weren't as consistently strong as their counterparts a decade later,
- the field was deeper in the 1990 s then ten years later,
- surfaces were less similar in the 1900 s
How can you tell wchich is the case with the data you showed?