The Nimbus fonts are PostScript fonts and normally the right replacement for the group Helvetica, Times, Courier, because those are Type1 fonts, too. Unfortunately Helvetica is then aliased to Arial, which would prefer Liberation Sans but that is not available, so Nimbus Sans gets aliased to it.
The reason for the aliases is that in some cases like Word processing metric-compatible font substitution is desirable. A line in Times New Roman should have the same length and height as in the substitution font. Otherwise you get garbage documents. Deja Vu is not metric compatible to the "classic" group.
Now for web browsing having identical metrics should not matter as much as hinting. Is the Liberation set better hinted? It's in Multiverse, but you could install it. You can also try if you like Nimbus better with the autohinter set to "slight".