There are quite a few different methods of calculating PEG value. For instance, PEG Ratio - is a valuation metric for determining the relative trade-off between the price of a stock, the earnings generated per share (EPS), and the company's expected growth. The question is how to define "company's expected growth". Many financial web site are using analyst's projected growth. In stock2own we are using most conservative approach and estimate "company's expected growth" as an average for the last 5 years.
On top of that, some web site are computing "trailing" PEG, which means data (earnings per share) for the last 12 months used. In stock2own we are using more "long term" analysis and annually reported earnings per share numbers are used.