It must be the mixing of memory.  Many varied results when mixing brands/types as previously mentioned.  Best option is to only have one brand/type of memory.  
Hello, my Inspiron 3670 had I7 8700 cpu and 8GB DDR4 2666 ram. I just added a second 8GB DDR4 2666MHz ram. In the bios under system information i see memory frequency 2666, but windows says it's 2400. Before the adding, with just one memory, Windows said 2666. Everything is working, i have my 16GB and the computer is working well, just wondering why Windows tell me frequency now is 2400. The memory i've added is HyperX Fury DDR4 8 GB 2666 MHz. Thank you
Inspiron 3670, ram frequency