If you're using Dynamic VCore, then of course your VCore will rise under full load—it has to compensate for Vdroop. How else would it maintain stability? Also, some chips simply require more voltage than others to run at certain frequencies; maybe your 2500K just needs that much voltage.
Judging from what you're giving us, 1.381V is seemingly the stable voltage for your CPU under full load at 4.5GHz. Please give us more information on your overclock. That is, provide other MoBo settings like QPI/VTT, CPU PLL (if used), DRAM frequency, DRAM voltages, DRAM timings, stress test program, duration of stress test.