Load64

Hi everybody, I just recently started my adventure in the world of preservation, buying a 1530USB.
I toyed with some tapes after testing the functionality of them on my trusted c64 and had some mixed results.
For the most part, if it works on real HW, the TAP also works, but I get some reports from finalTAP which I can't properly understand...

Let's talk examples:
Space Harrier from Encore (budget EU version) - it worked like a charm (also used POKEs to advance A LOT in the game) but finalTAP reports a "51% recognized"
Header test : PASS - Recognition test : FAIL - Checksum test : PASS - Read test : PASS - Optimization test : PASS

Mario's bros 2 (which is actually the ocean version of Mario Bros) by the notorius Armati brand* reports 23% and still works fine
Header test : PASS - Recognition test : FAIL - Checksum test : FAIL - Read test : PASS - Optimization test : PASS
I get the recognition fail, since it's just a pirated version, buth why the checksum fails and the game plays?

I'll add that both are single load programs, if that helps.

Even the Gryzor demo on the back of Combat school is a single load (78 laps for a single level...) and passed all tests - strange, because when I tried to load it on C64 it went out of memory error 

Last, Combat school itself: it didn't load on the real HW, nor did the TAP, but I still get a fascinating 94%
Header test : PASS - Recognition test : FAIL - Checksum test : FAIL - Read test : PASS - Optimization test : PASS

The question boils down to "am I doing it right?" before I have to start over the work 
Also, any guide, documentation, threads you can point me to will be appreciated.

*Armati unprotected games and sold them in Italy way before copyright laws were extended to SW