Why does blockchain.info custom transaction insist on a bigger fee?

[Jace]

I'm trying to create a custom transaction (two input addresses, one output address + one of the inputs as change) from my blockchain.info wallet, using a 0.00002 BTC fee.

But for some reason, it insists on including a 0.0005 BTC fee or something. I already changed the fee policy to 'Frugal' but it still happens. Why is this? When creating a custom transaction, why can't I customize it exactly the way I want?

(yes I realize that with too low or even zero fee a transaction may not be processed by miners, and I don't actually care about paying 0.0005 instead of 0.00002 or whatever, just wondering)

[Jace]

Note: it *does* work when using only 1 input address (and change returning to the same address). In that case I can specify any fee I want. But with two addresses, nope

[deepceleron]

You may be reducing the priority of the transaction to the point where it is below the relay threshold by adding to the size.