I took the example hash from your issue, and ran two different tests. 1 Adding up the block size, which resulted in:
```
calulcated ref size 15734625
expected   ref size 15729480
```

1 adding up the datasize which resulted in:
```
calulcated ref size 15729672
expected   ref size 15729480
```

the "expected" ref size is the size according to your issue