What I am suspecting, but I haven't actually seen this confirmed in articles, is that the heightening of the pH in a bleach / redevelop thiourea (or sulfide toner), actually causes more of the formed silver / ferri/ferrocyanide complex to be broken down and also converted to Ag2S.
Silver / ferri/ferrocyanide by itself is a light yellow coloured substance, as can be witnessed during the bleach. It is formed next to AgBr during the bleach, this latter substance being somewhat light sensitive. The silver / ferri/ferrocyanide is probably pretty stable though, but not in highly alkaline environments, see the referenced article here:
http://81.207.88.128/science/photo/toners/toner.pdf
It thus may explain the more yellowish tones of a plain thiourea redeveloper, without added NaOH, as the substance remains partly in the final toned print, while heightening the pH using NaOH, may help in converting it to Ag2S as well. Please note that this is just my theory based mainly on what Wilco wrote in the above article and other stuff I read.