It's probably due to the ratio of cement to sand. If 2:1 ratio of sand to cement or 2 sacks of sand to one bag of cement is strong, meaning, 4:1 ratio or 4 sacks of sand to one bag of cement is weak. One factor that contributes why this sometimes happens, is the very limited budget. Or, the contractor wants more gain than the good outcome.
Or, it can be due to sea sand that is fresh is used rather than the river sand. That's what I heard from experienced masons or carpenters. Thank you @phoenixwren.