Recall from the conventional commentary on the recent ECB rate change: Draghi May Enter Twilight Zone Where Fed Fears To Tread(Bloomberg - Jun 27, 2012)

The deposit rate traditionally moves in tandem with the benchmark, which policy makers kept at a record low of 1 percent on June 6.

This is because Central Bankers like nice, symmetric "corridors" around their target rate. There is no reason that the corridor could not be narrower than it is. According to Fullwiler

In some cases, operating procedures aimed at improving the central bank's ability to offset autonomous changes to its balance sheet or accommodate bank demand more precisely have been employed to reduce volatility (some examples related to the Bank of Canada are provided in Principle 8). Combined with a narrow "corridor" between the rate paid on reserve balances and the penalty rate, these tactics can be particularly effective in nearly eliminating deviations from the target rate. But while the former are consistent with the central bank's duties of accommodating the demand for reserve balances described in Principle 4, there are even simpler procedures for effectively eliminating volatility that are consistent with the principle that potential volatility is determined by the width of the corridor. For example, Fullwiler (2005), Whitesell (2006), and Lacker (2006) independently propose that the central bank set the target rate equal to the rate paid on reserve balances while leaving a substantial excess of balances circulating.

This seems to be what the ECB is, de facto, doing since 2009.

Draghi said "a few" officials called for a cut, fueling speculation the bank could act next month.

ECB Liquidity

The deposit rate has served as the de facto benchmark, steering overnight market borrowing costs, since the ECB started to provide banks with unlimited liquidity after the collapse of Lehman Brothers Holdings Inc. in 2008.

As we discussed back in February 2011...

That policy removed the need for banks to borrow from each other to meet their reserve requirements, pushing down interest rates. The euro overnight index average, or Eonia, stood at 0.33 percent yesterday.

Therefore,

"If you want to ease monetary policy, you won't get it from cutting the main refinancing rate," said Nick Kounis, head of macro research at ABN Amro Bank NV in Amsterdam. "Reducing it alone wouldn't translate into lower market rates. Slashing the deposit rate makes more sense."

More Fullwiler:

Given the difficulties in money markets beginning in the late summer and fall of 2007, Mosler (2007) and Goodhart (2008b) propose going a step further and set both the penalty rate and the rate paid on reserve balances equal to the target rate, creating a corridor equal to zero, at least (in Goodhart's proposal) for desired balances equal to some (policy-determined) percentage of a bank's retail deposits. In the context of substantial market unrest, Mosler (2007) argued that "when the [central banks] fully understand their own monetary operations . . ., they will offer funds at or just over their target rates and also have a bid for funds at or just under their target."

If you are not convinced, try it on someone who has not been entirely debauched by economics. — Piero Sraffa