But that's what the Scientific Method is all about. Leave enough room - in terms of saying maybe - just in case you need to CYA later if the current new evidence proves even slightly (like .02%) suspect later down the line.
Kinda like making a 50/50 bet so you never seem to collect the $200 & Pass Go.
Or risk becoming a Meteorologist (just kidding!)
Patience....patience. The scientific method may seem boring and/or slow but it DOES work........eventually.
Take a look at the overall data of past & current Mars surface missions. All have chemically analyzed soils/rocks in about the same way. Viking I & II were specifically sent to look for microbial life in the martian soil - and had very interesting results in 1976. Ultimately most scientists agreed that life "most likely" - again a hedge 50/50 bet - did not exist on Mars based on the Viking experiments back then.
NASA Curiosity project scientists want to be sure about their results as scientifically as possible - and also to CYA just in case. Remember the Mars Allan Hills 84001 meteorite/fossil debate in 1999?
One last note: spending 2.5 Billion on a "super rover" to Mars - you would think that any and all possible "contaminants" like Earth-organics would be a non-argument. Especially how NASA completely sterilizes anything going to the outer reaches of the Solar System like Jupiter, Saturn & Pluto. Let alone a planet like Mars - the most Earth-like in the Solar System that early-on once had oceans like Earth.