uvXYZdabcXYZfg
^        ^   
|        |
A        B
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The engine than starts to match at B + 1, and finds no such a match.

In the second case, the engine starts from the left as well, but finds no match:

uvXYZdabcXYZfg
^
|
A
[download]

So, it moves to A + 1 (still no match), and then A + 2, where it can match with (abc)* repeating zero times:

uvXYZdabcXYZfg
  ^
  |
 A=B
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After matching, it continues (because of /g) to B + 3 (no match), and at B + 4 it finally succeeds with

uvXYZdabcXYZfg
      ^  ^
      |  |
      A  B
[download]

Better now?

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]