SEATTLE — Federal data show he number of live marijuana plants eradicated in outdoor and indoor grow operations has dropped in most states over the past three years, while the amount of bulk processed marijuana seized has doubled in that time.

And authorities can’t pinpoint exact reasons.

One thing is known: California, which provides the lion’s share of the millions of plants eradicated every year in the United States, saw a 46.5 percent drop in plants eradicated between 2010 and 2011, bringing down the nation’s overall numbers.

‘‘You can’t attribute it to one factor,’’ said Casey Rettig, spokeswoman for Drug Enforcement Administration in San Francisco.

Shifts in tactics from growers, weather patterns, and budget cuts to local and state enforcement agencies have played roles in the significant decrease in eradication, DEA and local officials said.

In 2010, authorities seized 10.32 million marijuana plants from outdoor and indoor growing operations, according to DEA data. By 2011, that number had dropped to 6.7 million plants — a 35 percent decrease. Those numbers are amplified by California, where 7.3 million plants were eradicated in 2010.

The number dropped to 3.9 million plants in 2011, a 46 percent decline.

In that same time span, 37 states saw their eradication results drop. Data for 2012 is not yet available.

One of the most dramatic shifts came from Idaho, which saw its eradication results shrink by more than 98 percent between 2009 and 2011 — from 77,748 plants to just 786.

Although, the Caribou County sheriff’s office reported raiding a farm in southeast Idaho with 40,000 plants this week.

In order for a marijuana plant to be counted, it has to have a root structure, even if the plant is just inches tall, and be mature enough to yield the buds that contain the drug, said Doug James, Drug Enforcement Agency spokesman for the Washington, Oregon, Idaho, and Alaska region.