The sectors were not reallocated; the LBAs were reallocated.
Speaking with regards to your model of drive:
SMART attribute 0x05 shows clearly that you have 3 LBAs which point to alternate/spare sectors on the drive (i.e. instead of LBA 12345 pointing to sector 12345, LBA 12345 now points to sector 83982925782).
At some point during the drive's lifetime, it encountered 3 anomalies either during reading or writing. If during reading, an I/O error would be returned, SMART attribute 0xC5 would be incremented, and the LBA would no longer be readable until a write was issued to it. I refer to this behaviour as an LBA that is "suspect". MHDDs only do analysis of "suspect" LBAs when a write is issued to the LBA in question.
If during writing, the drive did the necessary analysis and determined that the LBAs in question pointed to sectors which could no longer be used (and that includes ECC being unusable), thus remapped those LBAs to spares. It did all of this during one event (tracked by SMART attribute 0xC4), rather than across multiple events (meaning, the write to the disk was probably in an amount that spanned multiple LBAs). When a remapping occurs, attribute 0xC5 is decremented.
SMART attribute 0xC6 is zero, which indicates all remappings were successful.
Your drive also tends to park its heads a lot (over 123 thousand parks with only a 9200 power-on hour count). If this is a 2.5" drive (e.g. laptop drive), this is fine/normal/acceptable.
HD Tune's author makes the assumption that a non-zero value in attribute 0x05 is worthy of showing the row in yellow -- that's the choice of the author, while SMART itself shows the attribute has not tripped (i.e. drive is still healthy when compared to vendor-defined thresholds).
Finally: going forward, you should not use HD Tune 2.55; this software is quite old and is very buggy. You should use HD Tune Pro, even if only the trial version.--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.