Perpetual bond pricing

You'd have to look at the details of the bonds to see any other differences (specifically the vague reference to "variable"), but there could be several factors that could make the lower coupon more expensive:

• Fixed vs variable interest rate
• being callable or putable (callable bonds are cheaper, putable more expensive in general)
• being convertible (e.g. to the issuer's equity)
• credit risk of the issuer (a bond from an issuer more likely to default will be cheaper)

The first three items are available from the prospectus - the 4th is not directly measurable and must be inferred from prices of other bonds guaranteed by the issuer.

The timing of payments shouldn't make that much difference, especially for perpetuals. The only way this would make a difference is if there's a huge jump in the risk of default between the interest payments of the two bonds, which would be unusual.