Alzheimers is a disease that can only be diagnosed correctly, after you are dead. It is often miss-diagnosed. Many of the symptoms are common to other neurological diseases, such as dementia, spongiform encephalopathy, and many others.
High levels of aluminium were found in "some' alzheimers sufferers, after death, when they could be diagnosed. High levels are also found in people where there is no neurological pathology. Low levels of al were also found in alzheimers sufferers, yet they still had the disease.
Much medical research requires funding, from either governments or drug companies,(there is independant research going on, but this is not the norm). In either case it is expected that the researchers will prove their hypothisis. This is why the results are presented in such a way as to prove a link between aluminium and alzheimers, even though the same evidence can be used to 'prove' the opposite.