Why are mashed potatoes lumpy and sticky at times and not at others?

The individual potato cells are susceptible to breaking down based on the amount of starch in the potato. Higher solids (starch) potatoes have larger cell size and tend to not break down as easily when over mixed versus a low solids potato (such as a red or yellow skinned variety) which has smaller potato cells. Among russet varieties the Russet Burbank from Idaho has usually had the highest solids and lowest moisture so turns out dry and fluffy. In Idaho we are required to list the potato variety on the bag or box.