Well  I can't  figure out Alamy's pricing system.
Sold One image today Royalty-free, resolution 1458 x 972 for 10.14$ -  6.34$ for me and 3.80$ for Alamy.
Now according to Alamy the price for Royalty-free images is:

File Size    Pixels    Price
56 KB   170 x 113   € 5.00
153 KB   280 x 187   € 15.00
396 KB   450 x 300   € 49.00
958 KB   700 x 467   € 90.00
4.1 MB   1458 x 972   € 180.00
16.2 MB    2916 x 1944 € 235.00
28.8 MB   3888 x 2592   € 305.00

Did I miss something?

Yup, many forum threads on Alamy about how prices are being driven down by micros/general  competition.
Like iStock, Getty, no doubt et alia, big buyers can negotiate bulk discounts.
Check out Alamy's Discussion forum.

4 MB
1329 x 972 pixels
231 KB compressed
   $ 6.33

have one too

I asked Member Services and according to their statement these sales are coming from ONE very large client who exclusively gets special discounts like the ones we encountered.

Alamy always offered discounts (I bet as most traditional agencies anyway...) to large clients but this one supposedly is a very special one that they don't want to lose.

That always bothered me - while the agency get the entire 40% of the sales the 60% chunks are divided into the many thousands of individual contributors. So of course, they still make a good bunch of money but we as individuals get the diluted soup.