It sounds like your Core i5-4200U is running exactly as Intel designed it to run. The instant the second core becomes active, the maximum multiplier immediately drops to 23. When a core enters one of the low power C States like C3 or C6, the remaining active core is allowed to switch back to the 26 multiplier. When lightly loaded, this multiplier switching is going on hundreds of times a second. Most monitoring software samples the CPU once per second and misses what's really going on. ThrottleStop uses high performance timers within the CPU and can accurately track exactly what each thread is doing.
Try running a single thread of the TS Bench test. During this test you won't see the full 26.00 multiplier. The reason for this is that the benchmark is keeping one core active 100% of the time. As soon as Windows needs to process any background activity, it is forced to wake up the second core which drops the maximum multiplier. When Windows is done with its task, the second core goes back into C3 or C6 and the multiplier goes back up. ThrottleStop should show a multiplier in the high 25 range depending on how much background activity you have running but it won't ever show the full 26 multiplier during this test due to Windows. CPU-Z might show the odd flash of 26 during this test but it usually just shows 23 to keep things simple.