Discrepancy and weak spots in concrete is mostly due to lacking uniformity in raw material. This could be thanks to different supply source, thicker and thinner consistency being mixed from the same supply source and also even mixing cement from different areas in the same quarry.
Differences in details of the process of manufacture and above all, the variation in the ash content of coal used to fire the kilin, contribute to the variation in the properties of commercial cements. Due to variation in cement strength, the concrete made from these cement will also have variable strength. Requirements and site conditions own W/C ratio v/s compressive strength of concrete curve should be developed at site itself.
It is often observed that cement bags marked as OPC-43 Grade may really be containing cement of much higher grade. Site cement samples should be tested for its actual strength and other properties. There are instances where higher grade cement is being used even for low strength concrete, as mortar or even for plastering. This can lead to unnecessary cracking of concrete/surfaces. Specifically, low grade mixture will gain in strength beyond the 28th day, while higher grade concrete strength is maximized by the 28th day.
During initial setting period of concrete, the higher head of hydration can lead to damaging micro-cracking with in the concrete which may not be visible at surface. The situation can be worse when we tend to increase the quantity of the cement in concrete with a belief that such increase are better for both strength and durability of concrete.