The performance of an SSD is at its best when the SSD is new (out-of-the-box). As the SSD fills with data, performance will degrade. Performance degradation varies based on the workload applied to the drive. Benchmark tests are expected to drop as the drive usage increases.
You can make sure you SSD is working correctly by using the Intel® Solid-State Drive Data Center Family for PCIe Drivers, also, make sure TRIM is enabled or run the SSD optimizer with the most recent version of Intel® SSD Toolbox.
The Intel® Solid-State Drive 750 Series Evaluation Guide has details about how the SSD performance is measured, it mentions the tools, hardware, configuration and procedures used. The advertised performance from the Product Specifications (Page 8) is obtained using Iometer* with specific test parameters. The results you provided are from tools that use preset parameters, so the results will be different (higher or lower) from the advertised in most cases.