The three matrices from which you want to isolate the vitamins are different and the levels might be different. This might explain the necessity to use different methods.

Personal preference or experience with a certain method will also play an important role.

No. The methods of extracting fat-soluble vitamins in milk, meat and fish is the same, however, the methods of extraction fat itself from the samples are different. More accuracy in the extration of fat means more accurate measurement of the vitamins. It should be mentiond that for fat-soluble vitamins that are protocols which are not dependent on the source of fat.

The methods are always the same. The difference is in the nature of each sample (solid or liquid). For milk the sample is already ready for analysis, but for meat and fish it is first necessary to prepare your sample to render it in liquid form (this preparation which differs according to the analysis which one wants, Vit C, Protein, fat ..). But do not forget to always report to the dilution of each preparation (to calculate the true concentration).

For the fat, Rose-Gottlieb method is the universal method, it's very useful to determine the rate of fat and especially if you want to do a qualitative analysis on the nature of your fat (GC or TLC). because the fat is still intact in this method and it's very easy to achieve.