I just sold 2 images accepted an hour ago at Shutterstock. I didn't know they can appear so quickly in search after review.

What I've noticed is that new images appear in search results almost immediately but only if the results are sorted by 'Newest First'. If you sort by 'Most Popular' they are not there and you can prove this for yourself by noting that the search result number actually changes if you flick between the sort options (alternatively choose niche search terms that limit the results to only one page).

Same, I was only accepted yesterday and I've already had three sales which seems unprecendented in my experience. Why is it so different to istock and DT and FT? What is the catch with SS?

Buyer hoarding.

Since it's a subscription all-you-can-eat buffet site with daily download limits, a lot of buyers try to stuff their faces by downloading the maximum allowed amount. 

When they first join they probably download the best of what they need. After they run out of stuff with the existing collection they then search daily by newest and download as much as possible from the newly added stuff. Every day.

That's why you get immediate sales for new stuff. But what does that tell you? You need to constantly upload new stuff or your sales will probably stagnate.

Some people here claim they get great sales without uploading new stuff. I'd say they're the exception to the rule and are lucky/talented enough to have some best sellers at the top of the "Most Popular" search balancing out the lack of new stuff. You'll see references here to "feeding the beast".