Vocabulário: To say, to talk, to speak

Dê uma olhada neste gráfico comparativo da frequência de uso desses tres verbos em inglês, entre os anos 1800-2000: http://books.google.com/ngrams/graph?co ... g=0&share=

A minha questão: você tem alguma teoria que explique as variações através do tempo?

This is not a trick question. I have no idea. Feel free to speculate, conjecture, propose, theorize, hypothesize or other. Talk is cheap. It's your chance to shoot the breeze.

Maybe because we can use "to say" meaning "to speak" or "to talk" depending on the context, but you cannot say "to talk" or "to speak" when we mean "to say"? I'm just guessing... I have no idea why, and I'm probably making a dumb statement .