Nurses, docs, ect, will most often give glute shots higher & more outside than the injections pics depict. My guess is that if you inject as the picture shows, you are much less likely to encounter the pelvic girdle---injecting higher & to the outside is generally less painful, but requires some knowledge of the human anatomy to execute properly. Here is a description re: this location I took from Anabolex.com:
The iliac crest serves as a landmark for this quadrant. The spot for an injection in an adult is usually to 7 1/2 centimeters (2 to 3 inches) below the iliac crest. The iliac crest is the top of the pelvic girdle on the posterior (back) side. You can find the iliac crest by feeling the uppermost bony area above each gluteus muscle. The upper outer quadrant is chosen because the muscle in this area is quite thick and has few nerves. The probability of injecting the drug into a blood vessel is remote in this area. Injecting here reduces the chance of injury to the sciatic nerve, which runs through the lower and middle area of the buttock. It controls the posterior of each thigh and the entire leg from the knee down. If an injection is too close to this nerve or actually hits it, extreme pain and temporary paralysis can be felt in these areas. This is especially undesirable and warrants staying as far away from this area as possible.