Erratic Critical Reading Score?

<p>My critical score ranges from 640-780. A few months ago I took a practice sat test and I got 780 on critical reading. I took the actual sat last June and i got 660 on critical reading. Today, I just took an sat practice test, but this time I got 640 on critical reading. I also started preparing for the psat recently, but all the practice tests I took I got around 76-78 on crit reading. Is it normal to have like 140 score difference? (note: my essay score is around 11-12).</p>

<p>Are those practice tests from the same source? Personally, I stick to official resources as far as I can. That is, tests from the Blue Book and released tests.</p>

<p>where can i find the source of released tests and will there be any explanation of answers?</p>