Even leaving aside ascription as the organising principle of the cline of instantiation, the relation between a reading and a text is not the same as the relation between a text and system.
A ‘reading’ is metaphorical for Senser sensing Phenomenon.
So, unpacked, a ‘reading’ includes a reader, the reading process and the text that is being read.
So, variant ‘readings’ of one text include variant readers and/or variant reading processes.
The cline of instantiation, on the other hand, is merely a relation between (variant) texts and the system of which each is an instance. It does not include readers or the reading process.
Variation in readers and the reading process are distinct dimensions from the cline of instantiation, and, since they involve actual texts, these dimensions of variation intersect the cline of instantiation at the instance pole.