A squarewave has equal times for its positive and its negative. When you filter it with an RC the positive part will have the same duration as the negative part. But PWM is not always a symmetrical squarewave, sometimes its positive part has less or more duration than its negative duration causing a normal change in the capacitor-coupled DC output level.
A sinewave has no harmonics. Your PWM waveform is 329Hz and has harmonics at 658Hz, 987Hz, 1316Hz, 1645Hz, 1974Hz, 2303Hz, 2632Hz, 2961Hz, 3290Hz etc. If you calculate the RC filter to reduce 329Hz to half its level then most of these harmonics will still be there but with reduced levels so the output will not be a sinewave.