Why is the driver pose different from the application pose?

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For example, I have a position(1.0f, 2.0f, 3.0f) and a rotation (0.948324f, 0.317305f, 0.0f, 0.0f), Rotate around the x-axis by 30 degrees.

vr::DriverPose_t outPose;
outPose.vecPosition[0] = 1.0;
outPose.vecPosition[1] = 2.0;
outPose.vecPosition[2] = 3.0;
outPose.qRotation = { 0.948324f, 0.317305f, 0.0f, 0.0f };

outPose.qWorldFromDriverRotation = { 1.0f, 0.0f, 0.0f, 0.0f };
outPose.qDriverFromHeadRotation = { 1.0f, 0.0f, 0.0f, 0.0f };
outPose.poseIsValid = true;
outPose.deviceIsConnected = true;
outPose.result = vr::TrackingResult_Running_OK;
outPose.shouldApplyHeadModel = true;
// update pose to steamvr

In unity, I will get this result (1.0f, 2.024904f, 3.087058f) and a rotation (0.948324f, 0.317305f, 0.0f, 0.0f).