The post will normally tell you it takes 10-15 WORKING days for parcels to arrive from the US to Singapore. This is just a rough guage though, as parcels can sometimes take longer or shorter, give and take.
There are any factors resulting in the slow and fast delivery of a parcel, so it's really hard to explain why a parcel will arrive faster than the other. Generally, the USA is a large country, so a parcel coming out of one state can be further than another another. How big the city is also a factor, since big cities have higher volume of mails and have the economy of scale, while smaller places will have to consolidate their parcels before they ship them out.
Customs is also another consideration. Sometimes for unknown reasons, the customs will retain a parcel as they deem fit. That will increase time of delivery too. I hope this helps.