Ok the point is clear.
we take calcDim left and top just if we did not specified any top left in options.

I would it differently anyway,
i would not check options.left and options.top in the initialize just to give this.left and this.top null.
At that point, after setOptions, those are undefined.
I would pass options to calcPositionDimensions(options)
and check options.left for undefined there instead of this.left and this.top = null.

And this was my first thinking.

Now while i write, i'm asking myself... this is gonna break SVGS.
Did you test SVGS import with this modifications? no nothing.

Do you have a link to the situation that is creating the bug?

@asturur Thank you for your reply!
Sorry, I am not good at English, I wish to confirm your opinion first.

You suggest that
this._setPositionDimensions(options);
and
this.left should be set by options.left is undefined or not.
Is it right?

So we do not add anything in initialize. is useless if you think of it, just send the options in _setPositionDimensions, is less code to write, maybe more readable.
Setting null to top and left, is gonna break everything. I do not feel safe even if i know that 20 rows later this is gonna be fixed by another condition.
Let's just avoid it.

@asturur Absolutely! Thank you for your suggestion!
I fix it.

I write a unit test.

var path = new fabric.Path('M 100 100 L 200 100 L 170 200 z', { top: 0 });
equal(path.left, 100);
equal(path.top, 0);

This was failed at master build, it would not fail after this PR.

Is it better to create jsfiddle?

It would be the situation for confirming the bug.
https://jsfiddle.net/sapics/mq00xq3y/101/

https://jsfiddle.net/mq00xq3y/102/

Ok here is more clear, same Path, same top, just left 0 and left 10 and the path are misplaced.
In your fiddle the path are too much different to give evidence.

Thank you anyway, nasty hidden bug ( created by me for sure )

Thank you for jsfiddle. It is more clear, absolutely!

@kangax this PR is safe.

Long of time ago ( jun 2014 ) i changed path initialize to solve various glitch of svg import and drawing in general. The logic was 'isLeftSet' and 'isTopSet', i changed them with this.top = this.top || calcDim.top ( same logic for left ).
This was obviously wrong because it was discarding user options with top or left set to zero.
Also i was traslating calcDim.left and top to respect originX and originY even in situations where was unecessary because this.top and this.left are already set.

Added to https://github.com/kangax/fabric.js/wiki/Changelog-(draft)