About PIN diode attenuator

Hi, I'm now trying to analyze a PI attenuator circuit.

Above pic is an ordinary PI attenuator using PIN diodes.
Everytime I googled it, almost every configuration is like the above pic.

However, on the board of interest, the direction of PIN diodes is opposite.
The board used 'BAR 61' which consists of three PIN diodes.
The above pic is from the datasheet of BAR 61.

Could anyone tell me the difference between two configurations?

The difference comes from the supply circuit.When the diodes are forward biased then connected the resistances, the circuit acts as an attenuator with these resistances.
So the real attenuator consists of those resistances, the diodes act simply as switch these resistances.
The first picture is simplified principal circuit, it's not a real circuit.

Thanks for your reply. I have another question.
To pass through the series diode in the second picture, the signal has to be kind of 'negative signal'?
Because the reverse resistance of the series diode would be really big.

No, when the PIN diodes are forward biased with a DC current, they act as bi-directional switch.Don't consider them as ordinary diodes.