1. The native ISO of the Sony A700 is about 160. Accordingly, ISO 100 is in fact ISO 160 with +2/3 EV exposure bias.
2. ACR adds 0.75 EV "exposure" adjustment to the Sony A700 images. The issue at this point is not this is nonsense, but following this procedure, the adjustment with ISO 100 should be 0.1 EV.
The result is, that the anyway overexposed ISO 100 shots appear hopelessly burnt without further adjustment.
It is worth to note, that the A900 behaves the same way, and look what ACR is doing: +0.35 EV adjustment with all ISOs except 100, and -0.35 EV with ISO 100, i.e. ACR accounts for the ISO difference.
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