What do we mean when we say depreciation calendar must match GL calendar ?

The depreciation calendar in Assets module must have the same number of periods and each period must have the same length as in the GL calendar associated with the ledger attached to the asset book. The only exception is the adjusting period. If your GL calendar has 1 or 2 adjusting periods then you do not create those in your asset depreciation calendar.

Sample Setup:
DEPRECIATION CALENDARS:
FA_CALENDAR 1: 12 PERIODS
FA_CALENDAR 2: 4 PERIODS
GL CALENDARS:
GL_CALENDAR 1: 12 PERIODS
GL_CALENDAR 2: 4 PERIODS
LEDGERS:
LEDGER 1: COA1, CAD, GL_CALENDAR 1
LEDGER 2: COA1, CAD, GL_CALENDAR 2
ASSET BOOKS:
ASSET BOOK 1: LEDGER->LEDGER 1, DEPR CALENDAR-> FA_CALENDAR 1
ASSET BOOK 2: LEDGER->LEDGER 2, DEPR CALENDAR-> FA_CALENDAR 2