kms_setmode test failed

We encounter an issue while running kms_setmode with eDP display that support variable refresh rate. According to the messages in below. Seems eDP display fall into lower refresh rate (20826us frame time) during testing.

Testing: basic 1 connector combinations
Starting dynamic subtest: pipe-A-eDP-1
  Test id#1 CRTC count 1
    CRTC[80] [Pipe A] Mode: 1920x1200@60Hz Connectors: eDP-1[236]
Expected frametime: 16659us; measured 20824.4us +- 4.799us accuracy 0.01% [0.10 scanlines]
(kms_setmode:21956) CRITICAL: Test assertion failure function check_timings, file ../igt-gpu-tools-1.27/tests/kms_setmode.c:590:
(kms_setmode:21956) CRITICAL: Failed assertion: fabs(mean - expected) < max(line_time(kmode), 1.718 * stddev)
(kms_setmode:21956) CRITICAL: vblank interval differs from modeline! expected 16659.5us, measured 20824us +- 4.799us, difference 4164.9us (867.8 sigma, 309.0 scanlines)
Dynamic subtest pipe-A-eDP-1 failed.