Question about X ray and ERC length difference

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Hello everyone,
I am currently on the fourteenth day of precice feumr extension in South Korea.
According to the length controlled by the ERC device (0.25cm * 4 for one day), it should have been extended by 1.3cm.
Today, I went to the hospital for a follow-up visit and the X-ray test showed that the extended length was 1.1cm.
Is this difference normal?
Is the actual length increase measured by X-ray or displayed by ERC?

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I do not know but I asked chatgpt:
X-rays for bone length measurements typically have an accuracy within about 1% to 3% of the true length. This means that if a bone is 30 cm long, the measurement error could be about 0.3 cm to 0.9 cm.

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i usually lengthen and assume im lengthening 1mm a day but when xray comes out its shorter when measured with a computer.  so just wait for results i guess. i didnt do a machine lengthening so it can be different.