Dr. Dan Johnson is, I believe, the veterinarian who was on the Board at Oxbow at one time. His credentials as an exotics vet and researcher are extensive. His paper entitled, "Rabbit Calcium Metabolism, Bladder Sludge, and Urolithiasis" talks about the way in which rabbits (and guinea pigs) process calcium, and why an overload of calcium in the diet can be problematic. Although he does state that reducing calcium alone is not a cure-all (he cites other factors that can contribute to sludge and stone formation), he makes the point that, based on the unique way in which these animals process calcium, it's prudent to reduce the calcium load for those animals who are prone to producing sludge.
Rabbit Calcium Metabolism, Bladder Sludge and Urolithiasis (Proceedings)
Although this was written a decade ago and is specific to rabbits, the information is still valid and relevant when talking about how calcium is processed in both rabbits and guinea pigs. Interestingly, Johnson also mentions Ca:P ratio, which is something I have questioned over the years:
The unusual calcium metabolism of rabbits makes it essential to maintain a diet that is well-balanced with appropriate calcium concentration, calcium to phosphorus ratio, and vitamin D content. If too little calcium is consumed, secondary hyperparathyroidism and bone resorption may result, while if too much calcium is present, the risk of urolithiasis and bladder sludge is increased.
Maybe we can use this topic to share other scientific data on the subject. I will add that, earlier this year, I invited a representative of Sherwood to post something on GL that explains the 'science' behind their claims that high calcium does not increase the risk of sludge or stones in guinea pigs, but she declined---citing a strong bias on the part of most pet owners against feeding alfalfa.