Why do double digest (XhoI & SacI) of NEB use buffer 1 ? - (Mar/06/2012 )

I recently did double digest ( XhoI & SacI ).
I went the website of NEB to check what buffer did I use.
I found that use buffer 1 + BSA.

Now I have a question.

The activity of XhoI that use buffer 1 is 75,but that use buffer 4 is 100.
(http://www.neb.com/n...roductR0146.asp)

The activity of SacI that use buffer 4 is also 100.
(http://www.neb.com/n...roductR0156.asp)

Why does the handbook or website(http://www.neb.com/n...sp#.T1bpTIfxqYA)suggest to use buffer 1 for double digestion of XhoI & SacI ?
Why dose use buffer 4 to do that ?
Can I use buffer 4 to do that ?

MY NEB catalogue says to use buffer 4 for a SacI/XhoI double digest.

I was looking for http://www.neb.com/nebecomm/DoubleDigestCalculator.asp .
My NEB catalogue says to use buffer 1 for a SacI/XhoI double digest.
And the NEB web showed me information below down here.

Double Digest Recommendation(s) for SacI + XhoI:
Digest in NEBuffer 1 + BSA at 37°C.
At least one enzyme has < 100% activity in this buffer, so additional units of enzyme and/or longer incubation time may be necessary.

NEB is switching to Buffer 4 for most of its enzymes. They probably have not gotten around to updating the double digest table.

I got this information.THX 434 XD