The differential in the image has 8 bolts and 10 holes because it is an 8-bolt differential, and the two extra holes are likely for alignment or for use with a different model.
The number of bolts on the ring gear is a key identifier for a differential.
Differential Identification: Differentials are often identified by the number of bolts on the ring gear or the differential cover. For example, a "10-bolt" differential has 10 bolts on the ring gear and often 10 bolts on the cover, while an "8-bolt" differential would have 8 bolts on its ring gear.
Bolt Patterns: The number of bolt holes can vary between different models, even within the same manufacturer, and can be used to identify the specific type of differential. The presence of extra holes can be a result of a shared casting or housing design that is used for multiple applications with different bolt counts.
Purpose of Bolts: The bolts are designed to handle the forces of acceleration and deceleration, securing the ring gear to the differential case. The number and size of the bolts are determined by the manufacturer based on the expected performance and torque load of the vehicle.