One of my drives seems to burn and read worse than the other

My drives seem to act weird. One of them seems to burn and read worse than the other. Even though both are LH-18A1Hs.

I think it's normal for two identical drives
to behave differently.
Rarely, if ever are two drives exactly the same
in performance.

That's also my experience.

Should I return the one that reads/writes worse?
The first scan of the two following scans was performed with the upper drive, the other one just after the first one with the lower drive. The disc scanned is the very same.
Greatest difference seems to be the amount of PI errors.

Click me! This burnt disc was scanned in the upper LH-18A1H.

Click me! This burnt disc was scanned in the lower LH-18A1H.

The difference in quality index is 5. But I also have disc I scanned in the two different drives and their difference in quality index was about 16 or 20. Seems like the worse the disc is actually burnt, the bigger is the difference in quality index.

Not unless it's consistantly out of specs, IMO.
Yes, it has been observed here in the past,
a truly good burn usually produces good scans,
while marginal burns can produce scans of varying
quality.

Think I'll keep it now. Have a 2-year warranty either way. I ordered some Verbatim DVD-R 16x. Hope I'll get as good results as the people here.