What you observed is normal for the most common lossy audio codecs (AC3, DTS, MP3, AAC, Opus, Vorbis ..).
They add:
1. encoder delay at the start (usually 5ms for AC3)
2. some extra padding at the end because their frames have certain sizes, not arbritrary ones. (at 48 kHz every AC3 frame is 32ms long so your file ends up a multiple of 32ms)
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