Algae based foods can also have high protein. Sera Micron Nature for example is, if I am reading the ingredients correctly, over 50% algae based and contains over 56% protein. Second ingredient is krill. Spirulina, by itself, actually has a high protein content.
The difference could easily be by the source of protein... i.e. vegetable and algae proteins are less likely to cause failed molts as compared to a meat based protein. Meat based proteins may cause more deaths.
If anyone has a tank where the shrimp don't seem to be thriving, but they aren't exactly dying at a fast rate either, then I recommend taking a look at diet. I was in that situation myself, nothing seemed to make any sense and I was given the diet advice. It worked! I have since given this advice to others and some have said they've also seen an improvement in their tanks.
Like I said though, if you aren't seeing deaths, it may not be a big issue.