Thai Lunar Calandar---year Cycles

So I'm trying to understand the traditional Thai lunar calendar known in Thai asจันทรคติ Jun-ta-ra-ka-dti. As I had always understood, it was based on a combination of the Brahmin and Chinese systems. As far as the Chinese system, the 12 year animal cycle is integrated, which confuses everything.

One would assume then, that a new animal year would begin after Chinese new year (February/March by the Gregorian calendar). Commonly misused in the west, however, people count from January first, which has no relevance to the Lunar and astrological charts as it is based on the Gregorian solar calendar.

Anyhow, by the Thai astrological rules, a new animal year begins on the first day of the fifth month. This can be verified by this (Thai language) software website calendar database on translating Lunar to Solar visa versa:

lunar<--->solar calendar

This confuses me, so does anyone know why it is this particular date? Yes, I know about Sonkran which is considered the Thai new year, but it's not coresponding exactly. For example Sonkran this year was celebrated from the 13th to 15th. The first day of the 5th month was April 9th (the first night after the new moon ขึ้น 1 ค่ำ เดือน ๕)

In other words, April 9th was the first official day for the year of the Chicken as far as Thai astrology/calendar is concerned. But the first day of Thai new year was April 13th, which was more than a week before the full moon of the 5th month (April 23).

There doesn't seem to be much consistency. Logically, I would have thought that a new year cycle would begin on the first month of the lunar calendar. Why, then is the third month (for Chinese) and fifth month (for Thai). What about the original Brahmin year cycles. I understand they have also have yearly cycle which I thought was 12 years but I'm not exactly sure?

Can anybody clarify what is the logic of the Thai new year based on the lunar calendar (for it doesn't fall on a new or full moon, neither does it mark the beginning/end of any lunar month)?

Also, if anyone with background on year cycles for Chinese, Brahmin, or any other Lunar based calendars could clarify the logic and reasoning of not beginning a new year cycle from the first lunar month.

Thanks in advance