(from a TIFF file => BW=> the JPG you see)
1. From RGB to BW, using the greayscale mode in Photoshop.

2. Fron RGB to BW, using the desaturation option in Photoshop.

It's pretty obvious that the method no. 1 doesn't have the same number of greyscale tones as method no. 2.
(if you cant see any difference - download the photos and compare in i.e. Acdsee. You'll see a huge difference!)
If you have to choose between these two methods, use no. 2.
If you want an even more advanced way of BW convertion, please to go my website for additional info. See the Photoshop section.