I have a 3.3 Volt PCI ethernet card working on a 5 Volt PCI slot. How is it possible?

The card itself is an 5740 IBM 03N5444 Quad Port 10/100/1000 Base-TX Ethernet PCI-X Ethernet Adapter which IBM states is a PCI-X 1.0a adapter that operates at 3.3 volts.

The motherboard itself is an HP P5LP-LE (Leonite) which has only PCI slots. More details from the manual itself here:

There are three 32-bit PCI slots on this motherboard. The slots support PCI cards such as a LAN card, SCSI card, USB card, and other cards that comply with PCI specifications.

With that in mind, this answer on Super User addresses the issue of using PCI-X cards in plain PCI slots:

Yes, as long as the PCI slot is a 2.x or later, PCI 1.0 was 5v while PCI 2.x was 3.3v - which is electrically compatible with PCI-X.

So knowing that, it does seem that someone manually “hacked” and extra notch to allow the 5740 IBM 03N5444 Quad Port 10/100/1000 Base-TX Ethernet PCI-X Ethernet Adapter to be keyed as a “universal” PCI card.

What are the ramifications of this? Honestly, unclear. This site does give some insight; the 3.3 Volt designation and 5 Volt designation refers to the voltage level of clock & timing signals and not power supply values:

The PCI specification defines two basic types of expansion connectors that may be found on a motherboard — one for systems with 5-Volt signalling levels, and the other for systems using 3.3-Volt signalling levels. This specifies the voltage level of the various clock and timing signals, but not necessarily the power supply voltage. A particular card may require both 5-Volt and 3.3-Volt power supplies irrespective of its signaling-level voltage.

Also more details from this post on what 3.3 Volt versus 5 Volt actually means; emphasis is mine:

33 MHz cards run on 5 volt signalling. 66 MHz cards use 3.3 volt signalling. (They both are powered by five volts; only the signalling on the bus is different.) The first "notch" (keyway) in a 66 MHz card is in a different place than that in a 33 MHz card; this is the different "shape" alluded to above. One would think that this would prevent you from putting a 33 MHz, five-volt only card in a 66 MHz, 3.3 volt slot. Or vice versa.

So that basically means that electrically, the card should be stable. But clock & timing issues? That might be the rub. You say the card is working, but perhaps it’s being inadvertently over clocked? Thus causing issues? Or maybe it’s working because the HP P5LP-LE (Leonite) motherboard is properly throttling down the voltage on that slot to 3.3 Volts? My gut feeling is it would be the later. But still have not found any details in the PCI specifics of the slots on that motherboard other than the 32-bit quote above.

My advice? If you feel a bit unsure about this setup, find a plain PCI Ethernet card with 4 ports for a replacement. Should cost less than $50 or even less than that and will be more stable in the long run.