File API - Progress - Question about Partial Blob data

The specs says :

" It can also return /partial Blob data/. Partial Blob data is the part 
of the |File| <http://www.w3.org/TR/FileAPI/#dfn-file> or |Blob| 
<http://www.w3.org/TR/FileAPI/#dfn-Blob> that has been read into memory 
/currently/; when processing the read method 
<http://www.w3.org/TR/FileAPI/#read-methods> |readAsText| 
<http://www.w3.org/TR/FileAPI/#dfn-readAsText>, partial Blob data is a 
|DOMString| that is incremented as more bytes are |loaded| (a portion of 
the |total|) [ProgressEvents 
<http://www.w3.org/TR/FileAPI/#ProgressEvents>], and when processing 
|readAsArrayBuffer| 
<http://www.w3.org/TR/FileAPI/#dfn-readAsArrayBuffer> partial Blob data 
is an |ArrayBuffer| [TypedArrays 
<http://www.w3.org/TR/FileAPI/#TypedArrays>] object consisting of the 
bytes |loaded| so far (a portion of the |total|)[ProgressEvents 
<http://www.w3.org/TR/FileAPI/#ProgressEvents>]. The list below is 
normative for the |result| attribute and is the conformance criteria for 
this attribute"

What is the rationale for that? The result attribute should better 
contain for progress events the latest data read and not the data read 
from the begining that you could easily reconstitute while the contrary 
requires more work.

Use case: calculate the hash of a file while you are reading it.

Regards

Aymeric

PS: I did not test it in all browsers, but unless I am using it wrongly, 
the result attribute is always null for progress events.

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