That's the problem with any method used to try and determine information about the distant past: you have to make assumptions about your information-gathering method that may turn out to be incorrect. In the case of ice core samples, many scientists ASSUME that x number of layers corresponds to y number of years. Which is fine and dandy until some example comes along which throws that assumption into question. Consider, for example, the "lost squadron" of WWII. These planes were discovered over 200 feet deep under many layers of ice. Therefore, they must be many thousands of years old, right? Now, this doesn't mean that ALL deep ice structures are that young, but it certainly throws into question any conclusions drawn from ice core data. Now, this is my opinion, but I for one feel like many (not all) scientists continue to assume such methods are reliable and accurate. Perhaps they need the grant money. Perhaps they have devoted so much to their research that they just can't accept that all the time they spent drilling ice cores may have been wasted. Or (again, this is my opinion), perhaps they are climate change zealots who feel that they must convince the population they are right, regardless of the cost of their scientific integrity.