This answer will only cover the gates in the Milky Way as Pegasus and Destiny Gates are different. Also, there is contradiction in the glyphs as later seasons presented them as the ancient language not reference constellations. I am doing my best to work around that.
Simple answer, they are using Earth's dialing program and interface with at that point is fuller featured then the DHD system. However they never got arround to updating the representations of the glyphs to match those on the gate at Icarus Base. Not surprising since they never updated the software when they switched to the Antartic gate and it's . So we can assume that on Earth Dialer software the PoO is always the old Egypt gate's 39th symbol.
Also, if you watch the early SGC dialing interfaces they are filming the symbols being locked in the chevrons. You can assume that they are using some mechanical method to turn the inner ring to match glyphs to chevrons. However DHD's don't use a mechanical method. They instruct the gate to dial the glyphs and dials them on it's own. By the time of Icarus Base their dialing software can directly interface with the gate and have it dial on it's own without assistance. So we can assume that the software does a shown symbol to gate instruction mapping.
Myself, I vote for the Dialing Software contractors were shortsighted enough that they didn't think to include a way to update the glyph representations. No one wanted to pay to rebuild the software so they kludged in a symbol translator. And by the time of Icarus Base they could directly talk to the gates no longer needing to mechanically dial them and verify what symbol locked in what chevron and if they standardized on a single set of symbols they wouldn't have to retrain the staff for every world they are stationed on or update the interface.
But if that's not good enough here is the:
Long and more speculative answer.
It is never established that all the source glyphs are unique. Each gate has 39 glyphs, 38 are reference points and the 39th is the source glyph for that gate. That gives us 1,987,690,320 possible source gates. Unfortunatley the show does not give us an exact count of gates but some rough math over at Is there any indicator as to how many planets have Stargates? estimates between 20,000 to 200,000 depending on the person doing the math. Even at just just 20,000 gates in the milky way that is a lot of unique glyphs, 200,000 or more, right out.
So if there are too many worlds to each have a unique PoO glyphs we have a few options:
- The 39th symbol could be a model number, depending on when the gate was created by the Ancients they get a different symbol. Unlikely but possible.
- The symbols are malleable and Ra and the Goa'uld changed the PoO symbol to meet their own needs. Perhaps the Ancients PoO was and on Goa'uld worlds they changed it to suit their own purposes.
- All 39 symbols are PoO and you use the closest one to your gate as PoO. This would require a few things, the 38 glyphs would have to be malleable as a part of the DHD Correlative Update System. This way when the gates are moved or the constellations change the representation can also change. In this case the PoO is the closest reference point to the world the gate was installed on. 39 doesn't auto-update because the Ancients never foresaw a reason to move a gate to a different world. If they needed a gate on a world they would just build one for it. This would indicate Ra took the Egypt gate from a world in vicinity of the Icarus gate if they truly shared PoO symbols.