Strange behaviour when passing chained methods (methodA().methodB().etc()) as parameters

import org.joda.time.DateTime
import static org.joda.time.DateTimeZone

class EWUtils {
      static DateTime nowUTC() {
              return new DateTime(UTC)
      }
}

//Inside a test case
import static EWUtils.nowUTC
...
DateTime baseDate = nowUTC()
println "${baseDate.minusWeeks(1)} ${baseDate.minusWeeks(1).millis}"
println "${nowUTC().minusWeeks(1)} ${nowUTC().minusWeeks(1).millis}"

// The output
2008-04-16T18:40:09.698Z 1208371209698
2008-04-16T18:40:09.709Z 1208976009709