VideoReader vs. FFMPEG

I have an issue extracting frames using the VideoReader class (on Matalb 2014b). When I compare the extracted frame with the one produced with FFMPEG, for example, or with VLC player, I get a very big differences that might reach up to 15 values (in 8 bit).

I wonder, what can be a reason for this, and how can I be sure that what I read are the real pixels' values.