Demystifying the Malayalam Months

Yesterday on my way back home from work, I was stuck with a strange thought process. About months in malayalam calendar and how they were matching to the english zodiac signs.
Kanni was obviusly virgo , the virgin -the meaning was so obvious. Tulam was libra -the thulas or balance  which was the sign for libra.Meenam was of course pisces. It was a bit more difficult to match up others but here they are

Chingam-simham-Leo

Kanni-Kanyaka-virgin-Virgo
Thulam-Thulas-Balance-Libra
Vrishchikam-(Googling got me that it means scorpion in Sanskrit)-Scorpio
Dhanu-Bow-Bow&Arrow-Archer-Sagitarius
Makaram-(this was a difficult one)-(Sanskrit meaning-Sea monster)-(Creature with Goat head and fish body)-Capricorn
Kumbham-Pot-Vessel-Water Carrier-Aquarius
Meenam-Meen-Fish-Pisces
Medam-Mesha-Ram-Aries
Idavam -Just coudn't get this one, the sanskrit one Vrishabham of course coincides with taurus. But Idavam and vrishabam? coun't make any connection.
Midhunam-Couple- Gemini-Twins -May be the constellation was interpreted differently.
Karakatakam-KarKat-sanscrit for crab-Cancer

Then I realised yet another thing. these months were exatcly one month off from the English Sun signs