I guess it's just down to how skilled they are at making use of the PS3's architecture, take Tomb Raider Underworld for example, PS3 version ended up becoming superior, the 360 version is @ 1024x576p, where as the PS3 version is rendered @ a full 1024x720p, So the 360 version ends up suffering from the slight blurriness that is usually akin to PS3 ports, also the PS3 version has less jaggies, and better shadows and lighting, I'm not sure about the textures though, they look a lot more detailed on the PS3 but that could be because of the resolution difference.
I think what this goes to show is that end of the day both systems are very very similar graphically but the small difference we see between multi-platform games can usually be attributed to different dev's working on each platform and taking advantages of each systems architecture differently, sometimes we'll see the 360 ahead slightly, other times the PS3.
I know this is a sore point for some but I do believe that technically the PS3 is the superior machine and will be able to edge ahead on the GFX front but not by a large margin, and I think it's something that we're only going to really see from mainly first party games from Sony and big exclusive titles as those dev teams have the extra resources and time to take advantage of the PS3's architecture, on the multi-platform gaming front I can''t really see things changing much, tbh I think the 360 will ultimately win out on that front with the majority of multi-platform game being slightly superior on the 360.
Any way just for the record I have an Elite & PS3 but I will be buying the PS3 version of RE5.