Assuming every w/s in content targeting has probability of conversion,
a<> a, in placement targeted campaign
Hence, P(Cp) - P(Cc)> b - a
or P(Cp) - P(Cc) > 0, Which means presumed enhanced click quality,
Assuming, Number of clicks = N, in original content targeted campaign, limited by budget(B)
So, Cc = N x P(Cc), and
Cp = n x P(Cp), which from earlier deductions
Cp - Cc = n x P(Cp) - N x P(Cc)
Observation : n < N, occurs in a large number of cases either due to bid going higher which is normal because, limiting exposure of ad, you force them to appear on same span, which reduces P(cp) value too. and quality score too decreases. ad placed to choose context within the closed domain which is lesser relevant than what it would have chosen in the open domain case in content targeted campaign.
Cp - Cc < 0,
So, Placement targeting actually Reduces your Conversion rate !!!