So what's the deal? Conspiracy?
No. Here's the two simple reasons for the different appearance.
Reason 1 = Repair!
Stretching and flattening of the parchment cockling!
Scribes and Correctors of the Codex Sinaiticus
Herbert John Mansfield Milne, Theodore Cressy Skeat, Douglas Cockerell
British Museum, 1938
Chapter 10
Condition, Repair, and Binding of the Manuscript
Subheading 14.
New Binding
Pages 83-85
[Page 83] “When individual leaves were mended—and often more than a day was spent on mending one leaf—they were put between wooden frames having crossed strings stretched across them. The two frames were placed on a damp cloth, a second damp cloth placed over them, and the whole covered by a piece of waterproof sheeting. In this way the vellum leaves were subjected to a damp atmosphere without actually coming in contact with the damp cloths. After about an hour, or an hour and a half, the leaves became quite soft and limp and could be straightened out on the stretching-frames (Fig. 23). They were then left to dry in the frames, still under tension, and in a few hours could be removed and placed in the press without any danger of set-off.
By this process the leaves were flattened, and although they became slightly uneven afterwards, they remain reasonably flat. Now that
[[Page 84 had photograph]]
[Page 85 continues text] the cockling has been eliminated the leaves show a marked tendency to curl towards the hair side, but this is a property common to all* thin vellum, old or new.”
Another photo (unknown source) of the flattening apparatus with Milne's & Skeat's book.
So, here we have the first reason
why the Standard Light setting photo's (BTW, the default photo's displayed on the
codexsinaiticus.org website) show very little cockling at all is because the worst parchment cockling was professionally repaired, i.e. stretched and flattened. Thus difference number 1 with the circa 1938 (presumably) British Library photograph in the previous post.
Here's the second reason for the difference in the photos.
Reason 2 = Light Setting!
The standard vs raking light settings make a difference1
Now, let's go back to the codexsinaiticus.org website, and click the Raking Light setting button,

Let's compare the photo's again, and look to see if the
cockling looks any different to the Standard light setting!
Ah! The cockling is still evident today after all!
So, the 1938 repair work removed the very
worst of, but not all of the cockling, which is still evident in the modern 2000's photo's - under the right/raking light setting!
Conclusion?
IMO, both the
poor storage conditions in Russia, a tin box, and the damp humidity and near artic cold of St. Petersburg (built on a swamp BTW) had a visible effect on the state of the parchment cockling of the Codex Sinaiticus
between 1911 and 1933-1938.
No, artificial Tischendorf vandalism theory is necessary (nor possible because he was dead!) to explain the visible differences in the photographs, when the simple facts (repair work and photographic lighting) are known.

P.S. Notice the color difference just between the Raking Lighting (more white) and the Standard Lighting (browner) taken at the same time of the same folio above!