how bitcoin is deflationary currency ?
It's not, at most the term used is inherently deflationary.
Right now the amount of coins in circulation increases every day, basically we're printing coins in the same way other currencies, at a rate of around $27 million a day worth of coins, so right now there is nothing deflationary about it, but since at some point the supply on the market will not grow at all and even till then the adoption rate will/might outrun the issuance you can call it like that.
what will happen if bitcoin is considered as global currency, it will never fall more than 5%-10% ?
Bitcoin is nowhere near the stage where it can be called a global currency.
I keep telling people we're a long way from that and it is enormous space to grow, there are like ~ 20 million addresses with a balance that matters, 300k transactions per day, and only ~150 mils in the LN, those numbers would be low for an economy the size of Bulgaria, not a global one.
So by the time we reach global those swings of 5-10% will become less and less frequent, but swings of 5% are not that uncommon even for normal currencies right now. Remember last year when everyone said the EUR is going to die since it crashed from 1.04 to 0.95 /USD in a week?
