1. Different concrete mix proportion: There may be differences in the concrete mix proportion used in the construction of the sky pump and the ground pump. The different proportions of cement, aggregate and water in concrete mixture ratio will directly affect the strength performance of concrete.

2. Different construction technology: There may be differences in process parameters between ground pump and sky pump during construction. For example, different factors such as pumping pressure, flow rate and pumping distance will affect the uniformity, compactness and strength of concrete.

3. Difference of concrete curing: After concrete pouring, it needs to be fully cured to ensure its strength. If the curing conditions of ground pump and sky pump are different, such as the change of temperature and humidity, it may lead to the difference of concrete strength.

4. Differences in quality control: The quality control standards and management requirements of the projects where the sky pump and the ground pump are located may be different, including differences in concrete mixing, transportation, pouring and maintenance, which may affect the strength performance of concrete.